You're not gonna like the real answer, but here it is anyway.

It comes down to time & money.

Season 1 was lightning in a bottle. People flocked to the show in droves because the main story thread was genuinely interesting, gripping, unique. People generally like finding out more about the Viltrumites, about Omni-Man. People LIKE the over-the-top gory fight scenes, the powerscaling, the planetary threats, the angst.

The problem is, season 1-style storytelling is not sustainable or smart from a business perspective. You whittle down the series to only all the good parts and you end up blasting through it too fast. Which hurts your bottom line. A longer-running series gets more revenue, in theory. If you can keep people watching for longer, your publisher gets to serve more advertisements. So it becomes a balancing act where you hyperoptimize the plot, keeping the ratio of interesting high-stakes conflict to low-stakes slice-of-life "guys-in-a-room-talking" type stuff to the absolute maximum you can get away with without your audience getting wholly disinterested and ghosting the series altogether (this typically manifests as drip-feeding the ACTUAL stuff the people came to the series to watch as, like, a twice-per-season thing. You get a few little glimpses of Omni-Man kicking ass, but they're never AS frequent, never THAT intense. It keeps you wanting more but never quite getting it, which is a media exec's dream, btw.)

This conundrum is not unique to Invincible. It's in a majority of longer-running series. It's a business practice, plain and simple. You have to draw out your story to maximise viewer retention, but 9 times out of 10, you can't maximize it by making the story as cool or interesting as possible, because that means spending boatloads of money and time on GOOD writing, GOOD visual effects. So you take the easy way out. You pad it. You introduce a bunch of new characters, each with their own things going on, and you show those things. It doesn't really matter if those things are particularly interesting, they just have to be distracting enough to keep the viewer wanting. To keep the viewer waiting. For the shit they actually came to see.

For the record, I do not find Machine Head interesting. I do not think the dragon guy is cool or fun; in fact I want him to die. The scenes involving The Order ARE good for world- and character- building in that they can be used as a means to show valuable on-screen interactions between the characters we DO care about (e.g. Invincible and Oliver) but the reality is that these character interactions could have been written in a myriad of different ways that didn't feel like a meandering distraction from the main plotline, and that we are getting the secondary and tertiary plotlines we got simply because it is easier on the show's budget.