I dunno, company that sells digital content for young people buys company that sells other digital content for young people. I can see the synergy there. Epic Games Store and Bandcamp aren't that far apart.
And there's like 300 of them. With Steam, I had to buy my way into a Buridan's donkey position where I don't play anything because I have too much. My man Sweeney made me jaded for free.
They wanted to use it to sell music licences for games and media production and the like. But it never really worked out, so they've sold it to a company that already actually knows how to do that.
A good reason would have been potential "synergies" with Harmonix, which they also own, but I don't recall ever seeing anything about that. Collaboration between the company that sells music and the company that makes music games seems like a no-brainer to me.