I can see what you’re saying, but here is my reasoning.
UE4 was released as an -almost- free software one year ago. I have been a 3d artists for a while and as soon as Epic decided to make this excellent move towards “do your game and if it’s successful, give us part of the revenue”, it opened up the doors for a lot of us freelancers/lone-wolves to finally dabble around or do business using a professional tool. We did not have this opportunity a year ago.
So, it makes sense to target the three most common OS’s so the opportunity is available for the widest audience. Also, Linux is a decent development platform and when I was at a prestigious university, CS student had to write software that compiled on Linux just because it was a developer-friendly OS.
Also, I personally don’t see myself switching to Win10 anytime soon or ever. I know you’ll say “this is what people said about [insert windows version here]” but the invasive telemetry stuff, the forced updates, the prospective locked-down distribution model really push me away for good this time. I reinstalled Windows 7 SP1 and turned off updates because I don’t want the nagging Windows 10 icon or the backported telemetry updates. Steam arguably helped popularized Linux when it was ported, UE4 could do the same.
As others have suggested, Epic could at least let us buy stuff from the marketplace without requiring the launcher. There are easy steps they can take to be more Linux-friendly.