In the simplistic world of TV, women like strong, confident men. Chandler is neither of these. In his own words (to Janice) he's "hopeless and awkward and desperate for love" (The one with the metaphorical tunnel)
Let's not forget that one of the running jokes throughout the series was that Chandler was gay. That wouldn't stop women liking him but until they get to know him (hard to do when he's always joking around and never lets people see the real him), they wouldn't consider him a suitable partner.
Not only this but he's never been too good with the protocols of talking to women. As Rachel once said when she thought he was gay "but then you spent my birthday talking to my breasts so I figured maybe not"
If you look at the relationships he had in the show, not many of them that ended were his fault. Janice notwithstanding, he had Aurora who was married and had other boyfriends, Kathy who cheated on him, Joanna (Rachel's boss) where he chose his friend over a partner.