It's a mix of the usual NTR-reasons combined with how absurdly popular SAO got, even as someone who read the novels before the anime was a thing I wasn't expecting it to become such an absurdly big thing myself, so yeah, it got a ton of loud haters all around despite many other shows/characters that came out around the same time or afterward being way worse in the things they hated on SAO/Kirito for. Also, the girls being very hot/cute probably helped, the amount of overlap of people wanting to lewd the girls and to shit on Kirito gave birth to an absurd amount of NTR for the show