Why is it always lesbians?
Why can't there be a good bi protagonist sometimes?
There are some bi protagonists around, if you look for them, but there are pretty intuitive market reasons for them to be less common than either straight or gay protagonists. While there are lots of individual exceptions, there's a general trend that people prefer to read romances containing characters who (a) are the same gender as the reader and (b) aren't attracted to people the reader finds unattractive. Thus the high prevalence of straight romance novels; focusing on straight people ensures that (a) is met for all their binary-gendered readers, and that (b) is met for all their straight readers, and thus that the novels have maximally-broad market appeal. And, given (b), bi protagonists are pretty inherently going to have less mass appeal than either straight or gay protagonists; because straight protagonists can fulfill (b) for straight people and
also for bi/pan/etc. people, and gay protagonists can fulfill (b) both for gay people and for bi/pan/etc. people, but bi protagonists will only fulfill it for bi/pan/etc. people, not for either exclusively-straight or exclusively-gay people.
There's a counterforce in that, beyond (b), people seem to additionally like reading about characters who
are attracted to people who they
do find attractive; so there's a market niche to be found in writing bi protagonists in order to appeal to bi/pan/etc. people who prefer them over straight or gay protagonists. But that's then aimed at a relatively small audience compared to either straight or gay protagonists, and so it's to be expected that running into those stories is going to be less common.
Of course, there are lots of authors who aren't writing for mass appeal, and are instead just writing things that they personally like. That's especially true on sites like this one where there's not much of a revenue stream. But you're still going to get fewer authors who like bi protagonists than who like either straight or gay protagonists, so the outcome will be similar even if the path is a bit different.