For this is very obvious reasons too.

Bisexual men are seen as "tainted" or as "gay". Like - "man one time with man == gay and not bi". While women can be how much they want with women and men would still see them as hetero. Hell, even lesbians are seen as "she just not found the right cock yet" women.

While bisexual men are despised by hetero women and by gay men. I saw too many bisexual men who were saying they are straight and afraiding to show their bisexual side - somethimes that was showing as homophobia, like "I am not one of those". Or if they liked men more - they were just doubling down on homosexual relationship and losing all hope to find a woman to date.

I don't know if it is everywhere in the world like this, but in places I have bisexual friends and where I live is like this (it is post-USSR, Japan, Korea and UK). In few cities I lived it was to the extreme, as bisexual man were believed to not exist at all by majority of people (so either straight or gay men, so if bisexual man once kissed a man - he is now gay man and always will be seen as one), while women are believed to be all bisexual but male leaning (so even full lesbian who lived 30 years with women only is seen as "male leaning bisexual").