I think it has to do with the fact that there is nothing special about being nice.
Consider an analogy of attending an athletics event and everyone is getting participation diplomas and only a few people get gold medals. Woman want the ones with gold medals, not the ones with participation diplomas, because all you have to do to get one is to show up, you dont actually have to be any good.
Niceness is by default expected by anyone. It does not make you stand out in anyway from other guys. Woman want guys that stand out, this requires you to have golden qualities such as a great sense of humor, good looks, muscles, intelligence, guitar skills, singing voice etc. Basically anything that makes you differ and better than her other choices.
Anyone can be nice, but not everyone is intelligent, highly educated, super good at playing guitar or fantastic at cooking etc.
You need to have other things going on about you that makes you attractive. Whatever that is will, hopefully, vary from girl to girl but there needs to be that something extra.