a) Because that has not been Mello's position, historically. This isn't a Birden/Kaine/Kerry/etc situation.
b) Because Bernie made it an issue when he full-throatedly endorsed Mello and another anti-abortion Dem but said "he didn't know" if Ossoff was a progressive. The juxtaposition of the two things immediately sent up red flags. He tried to fix the Ossoff stuff w/ a statement but lots of people had already noticed and were pretty pissed.
I think this is what the "why are people criticizing this?" crowd is missing. It's not that Bernie is compromising in campaigning for a historically anti-choice candidate, it's that he's campaigning for that candidate while, at the same time, labeling an economically moderate candidate in another race as not a progressive. Well, Mello is certainly no progressive on abortion, yet that doesn't seem to have stopped Bernie from helping out at all.
He walked it back and frankly Bernie's campaigning would be better served in Nebraska than Georgia anyway, but it's the hypocrisy that rubs people like myself the wrong way.