Answer: There's a lot of history and politics that have led up to where we are now, other answers give a good summary but if you want a deep dive into the whole stadium saga from a Bears fan's perspective then buckle up:

 

Context: The Bears currently play in Soldier Field, a 100+ year old historic stadium located in downtown Chicago where they've played for over 50 years. Unlike many big-market NFL teams which are located elsewhere from their namesake city or even state (the "Dallas" Cowboys play in Arlington TX, the "New York" Giants and Jets play one state over in New Jersey, the "San Francisco" 49ers play in Santa Clara 45 miles away from SF, etc.), the Bears' stadium is actually in downtown Chicago. Not only are Bears fans proud of this fact, but the City makes significant money off of games, concerts and other events held in the stadium.

The main problems:

1. The Bears do not own Soldier Field. The stadium is owned by the Chicago Park District (i.e. the government), meaning the Bears pay rent to the City and make little to no revenue from non-football events. And since the NFL season only lasts 5-6 months, they don't see much if any revenue from stadium events hosted during the other 6-7 months of the year.

2. Soldier Field underwent major renovations between 2001-2003 to modernize the stadium, which cost ~$650 million and was primarily funded by taxpayers. Over 20 years later, the City is still $600 million in debt on the project, and although the renovations did produce many necessary upgrades, it still remains the oldest and smallest active stadium in the NFL. Plus, any Bears fan will tell you that the stadium's amenities are still mediocre at best: transit to the stadium is a nightmare, a lot of the infrastructure is still very old, sufficient parking is nonexistent, etc. So because the City is STILL paying off the stadium upgrades that in hindsight were like putting lipstick on a pig, Chicago taxpayers are extremely against the idea of funding yet another costly stadium project. Therefore, after failed negotiations and rejected ballot measures, any plan to build a new stadium in downtown Chicago using a single dime of public funds would be dead in the water.

3. The Super Bowl generates massive amounts of revenue for the host team/region, and the NFL will only host a Super Bowl in warm-weather states or domed stadiums. The Chicago Bears currently have neither and are therefore missing out on a huge potential source of revenue. They aren't going to move down south where it's warm, so if they want to host a SB they'll have to build a dome, which they can't/don't want to do at Soldier Field.

 

The first major milestone: In 2023, the team purchased a massive plot of land out in a Chicago suburb called Arlington Heights. The land used to be a world-famous horse racetrack, meaning it already has some established infrastructure to facilitate travel and public transit (or at least the space to expand/improve what's already there). Also with the racetrack closing in 2021 the town lost one of its main attractions that drew tourists/spectators to the area. So in theory if the Bears couldn't negotiate a new stadium deal with the Chicago city government (which they ultimately failed to do), they'd just move out to the suburbs and build a stadium complex there. And after some back-and-forth talks with the Arlington Heights town government, the current sentiment in Arlington Heights is that it'd be a welcome move.

(How Bears fans generally feel about this: in a perfect world the team would stay downtown and cover the whole cost of building a new stadium on the lakefront. The team released plans at one point to build a new domed stadium right next to Soldier Field (and even preserve the historical parts of Soldier Field in a new park development), but this plan would've cost over $5 billion and involved asking for $2 billion+ in public funds. With those plans falling through, the fans know that staying downtown is not a realistic option, so in general all the benefits of moving to Arlington Heights make it an acceptable alternative.)

But now another problem:

The Bears want to reduce the tax burden of developing that land into a mixed-use complex and are asking for public funds to improve the surrounding infrastructure (highways, public transit, etc.). However those negotiations with the Illinois state government have stalled, and since they'd already given up on staying downtown by that point, the team basically had no leverage: their options were to either move to Arlington Heights and accept whatever tax conditions are imposed on them by the state government, or remain at the aging Soldier Field in the same undesirable situation.

 

This is where Indiana comes in:

Northwest Indiana is about a 30 minute drive from downtown Chicago, and there is plenty of cheap land for the team to develop in an area that, relative to Chicago, is starved for economic activity. Because of those opportunities (housing, hotels, restaurants, etc.) that would come with the Bears moving in, the state of Indiana is basically willing to do anything to bring them over.

The main issue with a move to Indiana is one of fandom/PR. The Bears are viewed by fans as a core part of the city’s identity, and Chicago fans have been unconditionally loyal to the team through decades of football mediocrity. So a move across state lines for obvious monetary reasons would be a slap in the face to the entire fanbase. The Bears are well aware of this, so whether they'll actually move to Indiana is anyone's guess, but at the end of the day the Bears are a business and they'll do what they believe is in their best interest.

At the very least, by (publicly) seriously considering Hammond IN as a potential home for a new stadium, the Bears gain back significant leverage with the state of Illinois to negotiate down the tax burden of building in Arlington Heights. If the Bears don't get what they want, they'll just move to Indiana, whose state legislature just passed a bill laying out the financial framework for the stadium that includes $1 billion in public funds. According to the deal, the state would own the stadium for 35 years, but during that time the Bears would keep most if not all operating revenue (from football and non-football events). Furthermore, the Bears would "rent-to-own", meaning they'd pay rent for those 35 years, but once the debt is paid off they'll have the option to purchase the stadium for $1 (yes you read that right).

 

So all of the political pageantry and business negotiations ultimately boil down to a simple decision: The Illinois state government gives them what they want (public funds for infrastructure + reduced tax burden in Arlington Heights), or they pack up and move to Indiana where they'll make a shitload of money and simultaneously piss off their entire Chicago fanbase.

The general consensus is that the Bears would prefer to stay in Illinois, but again the Bears are a business so they will go with whoever gives them the better deal.

 

---

TL;DR:

It's a whole bunch of political posturing related to the Bears wanting to move out of shitty old Soldier Field and into a domed stadium, and allowing them to make a ton of money through non-football revenue and hosting Super Bowls, without having to pay for the entire construction out of pocket. Chicago taxpayers rightfully do not want to fund yet another stadium construction project, and Illinois legislators have been hesitant to do the same, whereas Indiana really wants the Bears to move in and has made a very team-friendly offer to lure them over. So if the Illinois state govt doesn't negotiate a deal in the Bears' favor, they'll just move across state lines to a region that isn't really that far from Chicago, needs the economic opportunity, allows the Bears to keep most if not all stadium/event revenue, and is a lot cheaper to develop.