I cannot tell you why the Monsey bus is legal. I can tell you that one of the most famous supreme court cases ever says that segregation is illegal.
You are probably referring to Brown v. Board of Ed.
That case was talking about racial segregation, and it specifically limited its discussion to racial segregation in schooling. When that case was handed down, it was not considered to mean that all racial segregation was unconstitutional.
Also, note the difference between “unconstitutional” and illegal. Unconstitutional things are only prohibited for entities which are regulated by the constitution. (So for example, I am allowed to establish religion, and it does not violate the first amendment.)
The federal courts treat gender discrimination differently than racial discrimination.
But, besides for the US constitution, there are federal and state laws which also prohibit things.
I have no idea if this was illegal or not.