Most people either assume tomboys are lesbians so they don't bother pursuing them or well, some guys are just weird (don't want to ruin a friendship) or insecure (afraid they'll get made fun of), or who the hell knows, some people are honestly just a mystery to me.
It's the same principle as to how some guys only want girls with long hair, because short hair is like dating a dude. I... I just can't even...
Furthermore the kind of guys who are into tomboys may not be the kind of guys you're into. This fixation seems more predominate among betas and socially atypical males than your average joe.