From what I understand, "they" have already developed this, and trialed it. It worked great, but for many men it had certain undesirable side-effects, and for a very few the side-effects weren't good at all. But for most, it worked well.
The interesting thing? Almost all of the side-effects that were experienced by the men in the study all sounded exactly like the side-effects women experience when they are on "the pill"!
I wouldn't be a bit surprised if the reason we don't have a male hormonal-based contraceptive is actually because men can't handle the changes and problems women have been dealing with for decades.