Nice Guys are not nice
(inspired by Why "Nice Guys" are often such LOSERS)
As a reformed Nice Guy who has thought a lot about this I can say with conviction that the biggest problem with Nice Guys is that they actually aren't nice at all.
The Nice Guy attitude can be reduced to this: "If I let her cry on my shoulder she'll have sex with me."
In other words, the Nice Guy doesn't really care about the feelings of the woman he is interested in. He is just using his "niceness" as a ploy to get in her pants.
And women understand this, even if only at a subconscious level, which is why they never sleep with the Nice Guy.
As the article above suggests, the decent women run away because they know how the Nice Guy operates while The Bitches play along because it's fun to make Nice Guys jump through hoops.
Nice Guys need to get over themselves and need to learn to actually give a crap about other human beings besides themselves.