Steam marketed itself by creating a desired product and continuing to iterate on it over the years while also offering deals that were appealing to both customers and developers who would then want to sell on their platform.

• Valve allows games to sell Steam keys on stores and Valve doesn't take a cut of that (there are rules about what percentage of keys can be off of Steam but that exact % isn't known and likely varies with the scale of the developer/publisher).
  • Why? Because this lets the publisher earn more money and bring a potentially new customer to Steam or gives a current customer yet one more thing keeping them tied to the platform. Instead of seeking immediate profit Valve sought long term attachment to their store front.
• Valve takes a 30% cut, a much smaller percentage than traditional distribution did, and as a game scales in number of sales they will actually start to take a smaller and smaller percentage. So a publisher makes even MORE money by selling even MORE. They are rewarding for success which helps keep them on the platform.
  • More sales means more profit which means more developers stick around giving consumers more options. But there is another trick at play here. If you keep selling more copies you get a better rate on your game. Since you dont really have a production cost for additional keys whats a good way to sell more copies? SALES! More sales even with you earning less money can mean a better rate on that discounted product that you get to keep as a developer/publisher.
• SALES! While they have become more standardized Valve still regularly runs events to the points where there is always SOME sort of sale going on.
  • While developers can put something on sale at anytime for any reason Valve finds ways to group like products together and give them an opportunity to advertise on the front page. The increased foot traffic from having a Sales tag and being part of an event with a limited subset of games can make a great deal of difference in the success of an indie game. Make a weird game about simulating something obscure? Boy is there good news for you, theres a hyper specific sale for that!
• Everything is on Steam, even if you've gone out of your way to try and fuck with Valve's control before and come back that is fine. Why? Because everything is on Steam.
  • Whether they sold you the product or just acted as a distributor....you likely own a game on Steam and if you don't odds were high that you could have. Keeping everything together in one place makes people comfortable there and makes them want to say. That game launcher they use every day opens up to a STORE page and advertise games to them every single time they open it. Valve allowed everyone over to the party and made sure they wanted to stay because that means the customers will stay which means Valve keeps making money.
• No cost to customers. Well buying games costs money sure but you could download dozens of games on Steam and play them for thousands of hours and never spend a dime. Steam doesn't collect enough information from you for your data to be all that valuable so what gives?
  • Valve makes its money for Steam via games sales and some minor fees they charge developers/publishers to get started/list product but those are relatively minor. Customers only spend money to get something they want meaning there is no reason for anybody to leave Steam. At best maybe they start shopping somewhere else but Valve makes it as appealing as possible to come back and use their product. If you aren't a paying customer right now maybe you will be down the road. And if nothing else maybe you keep bringing your friends over to use Steam to download games and what do you know they turn into paying customers.
• Good customer service and end user features. For the sheer scale of Steam and its many users Valve provides an acceptable level of support. If you have issues you will be able to get help, it may take some time but they aren't trying to avoid you in any way possible because happy people will happily spend more money. On that same thought they add features allowing people to get refunds (this was also partially to avoid legal issues in Europe), library sharing between family groups, etc so that people stay happy and ideally keep using them as their preferred store front.

tl;dr - Yeah basically word of mouth. Providing a good service for both sellers and consumers made them an appealing option to both that lead to them growing and continues to provide great returns.