First, unfair as it is, male birth control is being developed in an era with much more stringent regulations about what kinds of side effects are acceptable. And, as one researcher pointed out in a recent AMA on Reddit, we know much more about the human endocrine system now than we did then. That means researchers have to test for many more potential physiological changes in men's bodies, resulting in many more potential reasons to hold up a trial. Researchers largely failed to look out for female patients while developing the first hormonal contraceptives, but that doesn't mean we should be that careless all over again. What's more, we've developed much better oral contraceptives for women in the intervening decades. The pills available today have fewer side effects than the original formulations available in the '50s and '60s.