QA takes time. Middleware may or may not be available. Launchers may not be there (especially in the case of Hearthstone). A lost of indie/kickstarted projects are still considered "Linux Scams" because they assumed that it'd be that easy.
It sure is, I'm a Linux-exclusive gamer ever since Steam on Linux came out, but I try to look realistically at reasoning of developers who never did this before.
Blizzard are well known for their approach of not releasing things until they're ready, and they won't want to kill that reputation for a Linux port.
This is the biggest concern. The linux community is crazy hostile. Heaven forbid your game doesn't work on some wonky setup that would be 100% unreasonable to support. We also see this in Android where reviews for games are sometimes, "Zero stars, doesn't work on $49 $obscure_chinatablet."
Even if a company picked a reference Linux distro, like the current Ubuntu, it would be still displease a lot people, especially if the game only worked correctly on closed video drivers. There's really no winning in such a fragmented environment, unless you're willing to invest a serious amount of your budget here. Its not just "push a button."
"Crashes when using wayland. FAIL!"