Because that's what MOST of the comments on the begged video were about...the "political correctness" of the new movie. Blah blah blah...they hated the gender change.
I think there were ways they could have won the fanbase over even with the gender change.
Ways they didn't care to try because they didn't respect the existing Ghostbusters fans and were more interested in trying to turn Paul Feig fans into Ghostbusters fans. Which is dumb, because you need both to support a tentpole film.
(1) They could have made this a SEQUEL with the new Ghostbusters (or at least one of the new Ghostbusters) playing the daughters of the OGs.
(2) They could have had Aykroyd co-write to tie the style of humor more closely to the original.
(3) They could have included the original actors playing their original roles as mentors to the new ghostbusters.
(4) They could have include Aykroyd, Murray, and Hudson heavily in the marketing.
They lost as soon as they decided that the opinions of the original Ghostbusters fans didn't matter.