I don't recall the exact sources of the details currently swimming in the smooth brain of mine but I believe/read/believe to have read somewhere that the Reds gentling male channellers over time decreased the population of potential channellers - so there were fewer, and weaker, ladies available to train. The stringent requirements for entry in the White Tower did not help matters in terms of actual trained channellers, but the abstinence and male-free environment may have contributed to preventing these women from having families.
I want to say that theory was thrown around by the Blues somewhere but I can't claim a source outside of RJ's somewhat tired Red vs Blue dynamic. "I read it in a book" isn't helpful when there's so many out there.
I view the onslaught of strong ladies of all ages suddenly appearing ready to train as similar to how all these men who wanted to learn to channel turned up at the Black Tower - the Wheel providing the Ta'veren with what they needed at the time. Rand is Ta'veren and these Soldiers came to him; I know there's some discussion about Egwene's pull on the pattern, or the lack of it, but I like to think similarly for Team Egwene as well. (That said, I 100% agree that the White Tower did itself no favours by not going out and finding enough girls, and being too strict with the ones they did have. I like to think the Black Ajah is responsible for this in some way, keeping the Tower from reaching its potential.)
The Aiel sending their male channellers to their ultimate demise (well) in the north of the Three-Fold Land probably has a similar effect.
I think the exceptional cultures in this question might be the Sharans using their male channellers as breeding stock, or those in the Isle of Madmen who are all as insane as each other regardless of gender. But otherwise I think everywhere else reduced their channellers because they don't know what DNA is.
Anyway I should listen to the episode to see if I'm just repeating what someone else has said!