Since certain things need to happen like last battle, DO prison being opened, people forget DO exists etc. But nothing saying the same thing has to cause it, As @bringbackthomsmoustachestates it doesn't have to be identical events. Maybe the Seanchan cull the ability by capturing too many women who channel, imean they capture the women and men who will channel regardless. If all they leave are the people who need to be taught, then would stand to reason in time channeling would be rare. Or maybe it's a sickness etc.
So many men are suddenly about becuase before no one was crazy enough to go about testing to find men who had the ability to channel. As described in the books Taim would basically go town to town and gather up anyone who even showed an interest in his offer.