And compulsion is way easier than Turning because you can do it with a single channeler and no Myrdraal. Honestly I think it would have been better if only channelers were vulnerable to Compulsion; non-channelers being immune to mind control would balance out a bit the immense advantages channelers hold over them in every other respect. In regards to why the Shadow doesn't use Turning more often, there are two possible reasons: 1) They'd have to get 13 Myrdraal into the Tower to do it to a lot of people at once, which isn't exactly easy 2) Most of the Black Ajah isn't all that into actually helping the Dark One. They're more focused on their own personal aggrandisement and, pre-canon, thought the whole Last Battle businesses would take place long after they were dead. Turning the Light-side Aes Sedai would actually make them less useful to the Shadow, because those Turned Aes Sedai can now replace them for most stuff, and why would they work to make themselves redundant?