It's a matter of self-worth and managing expectations. Most people who are attracted to conventionally attractive women are successful enough in life to pursue them, and spend their time in interpersonal relationships rather than on forums.
Conventionally attractive women, being the societal expectation of the preferred partner, have multiple options when choosing a partner. Options that men with low self-worth feel they cannot compete with. Ergo, they choose to pursue the woman who they perceive they will have less competition in pursuing.
There is also a matter of taste and development of attraction. Society and personal experience can influence what you find attractive. The common saying used to be 'gentlemen prefer blondes'. Regardless of the truth of the statement, it resulted in more women changing themselves to be blonde. If you have formative relationships with a girl of another race growing up, you are more likely to pursue women of that race growing up. And some people just have odd or niche fetishes due to inherent brain chemistry.