If you have Diablo III.exe, Wow(-64).exe and Starcraft II.exe on your system, do this for fun:
strings *.exe | grep -i linux.
Guess how many hits you'll get from D3? Yes, zero. So D3 is highly unlikly.
SC2 does have a hit, so it does seem possible, but still unlikly. (I forgot on the top of my head what this string was)
Wow however, actually has the following:
as one of the strings. So even the 2013 builds still have this functionality and it is quite likely a Wow client internally still exists and is used to this day.
So if we are talking about minimal effort to port, then Wow stands a good chance.
And the 'why haven't they released this years ago' ... is kinda asking the obvious. They didn't want to. While to us it seems obvious, they could release an unsupported client, it will still require possible extra support questions and thus cost blizzard's support staff extra time and thus money they don't want to spend.