(I won't mention which artists I think are rightfully on the list, and
which ones I think aren't rightfully on the list; with the exception
of Kurt Cobain. Nothing against him, but he couldn't sing. The others
I'd like to mention can be debatable, so I won't mention them).
I think that some of the artists who should be and are there were misplaced. Example: Joni Mitchell at #42, she should definitely be quite higher up there than #42. (She's not one of my favorites, but she's an excellent vocalist. Her melodies are complex, and they're
good at the same time).
Nominations and votes are not always the best description of the
criteria. There are other elements that could contribute to the
determination of ranking vocalists. Complexity of the melodies, the
styles (which makes it tougher to rank), the feeling that the vocalist
puts into their songs, the versatility, the melodies being set to
lyrics (the quality of the two, if they match well). There are several
types of criteria to use, which is why the list will always be
much different.

I read in a couple of magazines, the top 100 and 500 country songs of all time for example. One version would include songs that the
other didn't and vice versa. The songs that were included in both
versions were placed in completely different rankings. Rolling Stone
might do this again, and the list will probably be very much different.