"Maybe they did have a bad experience." Considering what had just happened, it doesn't seem all that unlikely that something terrible could've happened in the past that caused Thorne and the Free Cities to decide High Magic shouldn't be taught, like demonology or necromancy seem considered in Istredd's world. But more likely--

"Or it just didn't work for their agendas. If the leadership wants to be seen as some sort of true authority, they can't really be seen appealin' to an outside power for their magic." The psychology of this is easy enough to understand, and that's almost reassuring in a sense.