To be fair, Rolling Stone focuses of course, on popular music - not necessarily just pop itself, but you're not going to find obscure names on there.
Freddie Mercury should absolutely be way, way way up there, because the man could do everything with this voice.
Or, for example, why is there no mention of Whitney, Mariah, or Celine?
Of course the list is subjective, and mayhaps it's best split by gender or genre, but there are amazing voices that can and do span many genres. Michael's is one of them, and I agree it's underrated, and much more versatile than people know. However, Freddie was known for his powerhouse pipes and everything he could do with them.
This is why RS sucks!