Currently, it is far easier to learn how to properly employ the sweeping "gun vs bomber" attack then learning the proper range, aiming point, speeds, etc in which to deploy the German WGr21 air to air rockets. The WGr21 projectiles have a timed fuse and unless you are close enough the bomber when they explode, as in close enough for "puffy ack" burst to do damage, you are taking a lot of extra weight and drag (even after launch the drag of the tubes is quite noticeable), most people elect to not use them.
Timing the launch of the WGr21's is not easy. The speed of the bombers, the speed of the firing plane, and the trajectory of the rockets are factors that need to be dealt with, not to mention the actual range of the rockets.
I tried multiple times using the 109, 190, and 110 as launching platforms, using all the tips and tricks given by veterans of the weapon to bring down enemy bombers with the WGr21 rockets and found myself close, but not close enough. As I watched the films I could see parts of the enemy bombers being blown off, but I was not ever to hit an engine, cause a fuel leak, or do anything other then what I call "tertiary" damage.
Perhaps vs the much bigger target and defenses of the B29 the WGr21's will see a gain in popularity. But then again, they are even bigger targets to hit and the cannons of the 190A-8 or NiK2 "George" will do nicely, I'm sure.