Guys are attracted to dumb crazy and mean girls, and pursue these girls for sex.
When it comes to long term relationships, these type of girls are not "marriage material", so those of us who want to be happy, sane, and satisfied in life soon learn that the women you feel like you want to bonk aren't always the one's to take home to meet the parents, and the one's that are attractive and worth sticking with usually aren't so mean, crazy, dumb, or unpleasant.
If they're mean to someone else, chances are at some point they'll be mean to you, so not really worth going there.
Likewise if you aren't intellectually compatible i.e., if there is a vast gap in intelligence level between you, then it will be pretty hard to maintain the relationship.
If they have some sort of mental problem, then best to let them sort their problem out than to go there, unless you think they really are interested you, and in taking your concern seriously and willing to attend therapy as a couple.
There of course is also the possibility that some of the so called singe nice smart girls are not so nice themselves, or are more picky.
It takes two to tango, and sometimes the reason you're single is looking you in the face when you look in the mirror, and not the fault of someone else at all.