Teaching double backs is very difficult, and very few people in the cheer world have the experience necessary to teach them. A double back is not just pulling tighter, or punching harder off the floor. The set is completely different, as is the spotting technique. As a gymnastics coach I "studied" under another coach learning how to teach and spot them for almost a year before I was ready to teach and spot them on my own (and I was able to DO them on my own in my floor routine). I practice spotted on our elites for a good month before I started spotting them on my own.
I really feel that cheerleading just isn't ready, as a whole, to go there with tumbling. As mentioned above, perfection before progression. Also, there are just too many "self taught" tumbling "instructors" in the cheer world. There is still too much of a "the comp is this weekend, so chuck it even if it's ugly" mentality.