Perhaps you included this in your DRM comment, but I think the biggest issue is anti-cheat. Most online games with a sufficiently large 'competitive' element require a kernel-level anti-cheat which won't work on Linux.
Vanguard doesn't work, and perhaps that's for the best.
Then again, kernel-level anti-cheat is not that hard to bypass with special hardware, either. I guess someone ran the numbers and decided that blocking some percentage of cheaters at the cost of blocking 100% of Linux users was a worthwhile trade.