For one thing it's mentioned many times throughout the series so it's well documented. Beyond that it is far more perturbing that other things like more sophisticated versions of Healing died out. Weaves like these would have been necessary on practically a daily basis during the Breaking and with Strength in the Power not being a factor it should have been far easier to train far more women in these techniques. Traveling (barring those with strong Talents for it) requires a level of Strength that means most women would have to be pretty far along in their training before they could even make the weave work.

From what we have seen thus far the majority of women don't possess the Strength to create Gateways, even assuming our sample is skewed toward weaker women thanks to AS recruiting policies, there is still a Strength barrier. Couple that with the fact that even when Forced most women will be years into their training before they have developed the Strength necessary to complete the weave. It becomes easy to see how even a woman with significant strength and only cursory formal training would never be taught or learn the weave. It's not like they had time to sit in OP training classes day to day. Training for these women likely happened on the fly when opportunity presented itself rather than " today class we shall teach you how to Heal"

Think of how Moiraine approached Egwene in EoTW ... When they stopped for the night they ran through basic exercises and it still took even Egwene (clearly a fast learner) months to control even the most basic elements of embracing and channeling the Source. This is the most likely way women taught one another during the Breaking, and who is to say that the "Master" lived long enough or stayed in the area long enough to do more than teach the basics. I'd bet most AoL AS taught many women they happened upon the fundamentals and then moved on. Perhaps they recruited a handful who were willing to leave their families for one reason or another, but the vast majority of the women they would have encountered were probably too scared to leave their family/Tribe etc and run off to become AS.

Couple that with the Strength issue and it's very possible that Traveling became lost rather quickly when only a small percentage of women with the ability would have access or desire to pursue longer term training and there is no guarantee that those who were willing had either the Strength or a Mentor who lived long enough to teach them once they had achieved the requisite level of Power.

Not to mention there would have been very little need to Travel any distance. Why would an AS need to Travel vast distances during the Breaking? They would have no communications coming to them from other areas of the world and if they were searching for insane male channelers it would be far more effective to systematically search an area and clean out the men in a more military style. Using Traveling to search would be more than silly since it would require a staggering amount of luck to find such men. And With no permanent settlements where the hell would the Travel to?