Sad news. They have a 50 million (!) player base, 350k+ online right now. That means over a billion USD in revenue, probably way more with crates/keys taken into account.
Unless the cost of their server infrastructure got wildly out of control, why would a company that is raking in cash sell out?
Epic bought publisher of a highly popular game their users.
Psyonix sold out, because at everything is has its price.
Expect Epic to forcibly migrate Rocket League Steam users. They will turn RL Steam into an Epic launcher and all current RL Steam users are suddenly Epic users.
For Psyonix and Epic there is no real downside. If you care about this decision, you're not in their demographic, and probably didn't help them generate their F-You Money.
I interpreted parent comment as "you're not in [Epic's] demographic, and probably didn't help [Epic] generate their F-You Money", as a reference to the money gathered from Fortnite.
Unless of course you are referring to the paid version of Fortnite, but I don't think that's clear from your comment.
Unless the cost of their server infrastructure got wildly out of control, why would a company that is raking in cash sell out?