So Epic Games is buying Psyonix. Everyone's upset for a multitude of reasons, a lot of which aren't the right ones. A few very special people are even content with this news for the wrong reasons, which is even worse. Now, imagine the next three paragraphs as panels in the expanding brain meme.
Lots of people are just upset that Rocket League is going to show up on the Epic Games Store (EGS) and aren't reading any further into the issue. This opinion is not so bad because at least they're still doing the right thing, just for the wrong reasons. Also, it seems nearly universally agreed that the EGS is... not very good.
A lot of people are indifferent and just want everyone to shut up and move on. Obviously, this accomplishes nothing other than making the people who know what's going on talk more (see: this blog post).
Some people are actually supportive of this, their reasoning being that they "can still play it on Steam since they already own it there, and this will just bring the game to a wider audience". The last part may be true (summary: ~40% of EGS users come from consoles rather than being an existing PC/Steam player), but this still misses the actual issue at hand. However, I say "may" because Rocket League's playerbase is already mostly console players.
Very few people are seeing the actual issue at hand, however, which is that by removing the game from Steam, Epic/Psyonix (I don't know which is actually in charge of this decision, but if I had to guess I would pick Epic) would be engaging in extremely anticonsumer and anticompetitive behavior. It's true that neither party has yet to confirm or deny that Rocket League will be removed from Steam after it appears on the EGS.
However, if Epic/Psyonix wanted everyone to calm down, you'd think they'd just announce that Rocket League will remain purchasable from the Steam store and fully supported as it has been, and just also be available on the EGS. There would be no issue if this were the case, as then users would be able to choose whether to play on Steam or on the EGS, which is the ideal situation. The lack of such an announcement says to me that they've already decided internally to remove it from the Steam store and they just don't want to deal with the backlash from the public yet.
Some other issues that have not been addressed is whether current owners of the game on Steam will be able to buy Keys, Rocket Passes, or DLC cars once the game appears on the EGS. Whether Psyonix will continue to support Rocket League on Linux is also up in the air; since the EGS has no Linux version, it seems "reasonable" (i.e. from a business perspective) to me that Psyonix would stop ensuring that updates worked on Linux if Rocket League were made unpurchasable on Steam. Valve's Proton is very good, so that might be an option, but if it comes to that I'd bet a lot of us Linux-using software-vegans probably won't be playing the game anyway.
I'm not going to re-explain why exclusives are harmful here since this has already been done by several others at much higher quality than I could manage. If you're tired of reading, this video does a really good job of laying out the issue in terms of streaming services. This translates really well to game marketplaces since both are just DRM CDNs at their core.
TL;DR: To oversimplify, if you're annoyed with having to pay for multiple streaming services who are all lacking important features, you should also be annoyed with companies like Epic buying exclusive rights to having games on their launchers.