Why didn't the pope do something?
Pope Paul VI was not noted for swift action. I read once that the Italians described him as using two stones to kill one bird. At the time this happened Pope Paul had a committee studying ABC. He appears to have tried every avenue to find some loopholes before writing Humanae Vitae
a couple of years later. Then, when it was publicly rejected by many, he never took any steps to enforce it.
Several priests in Baltimore publicly rejected Humanae Vitae
and their bishop suspended them. When they appealed to Rome, Rome reinstated them and told the bishop to use a pastoral rather than judicial approach in such matters. This strongly influenced the selection of new bishops [and the attitudes of those hoping to be bishops.]
We are now reaping the results of that order. As one might logically expect, the bishops used a pastoral rather than judicial approach in dealing with erring priests in the next couple of decades.