'Cos that's how they were first designed for strength & stability. Later it was removed for the ladies and their long skirts and as they were more refined and genteel the bicycle didn't need to be as strong.
I guess once the bikes without crossbars became associated with "the girls", the boys didn't want to be seen dead on them, so the crossbar has remained the blokes choice, and then it comes down to what sells ....................

One vote for tonyc 

Edit: Plus it was unseemly for ladies to sit astride anything 

@JennyLeez

... Luckily I'm old enough to remember it ...

Ah, the good old days