You're assuming wrong: you can play with friends on different platforms
No, I did not assume that crossplay was impossible. My point is that the convenience is a feature. Let's assume I'm on Steam and my buddy is also on Steam. I have Rocket Arena already, he doesn't. Previously, if he saw me playing the game, he could just click on the game from the Steam friends list and buy it, then join me just as easily.
Now, though, he can't buy the game on Steam. He has to manually search for the game in a different store. He'll need to download and install EGS - which he may not be willing to do. And once it's installed, there's still additional hoops to jump through: cross-play is not enabled by default, and I'm not on EGS so at a minimum there will be an extra step to add me as an in-game friend or something. Again, I don't actually own RA, so maybe some of these details are wrong... but ultimately I believe the Steam version just became slightly less convenient for existing users.