Because it's not on the market. It also wouldn't be desirable if it was due to how it works.
The only way birth control works for men is lower their testosterone levels that in turn lowers their sperm count. That however makes them fat, moody, incapable of getting an erection, weak, and suffer from other signs of low testosterone. It would also negatively impact the development of teenage boys and young boys due to teenage boys taking it and from it getting into the water supply. In short, we know how a male birth control pill would work and have even tested it, but it won't hit the market due to how it works.
Plus, the pill would likely lower birth rates to about 0.5 or so making it quite damaging to the economy of the world. As it stands now the female birth control pill is lowering birth rates to a point where it is damaging the economy, so there is little reason to make the matter worse with a male equivalent.