Sorry, I don't have the answer. I just can't think of any possible reason. Emulators have ZERO threat on Apple's business model. Emulators are for running old programs, no one in his/her right mind would create new apps for emulators. Most of the programs are out of market, their companies even don't exist any more. Emulated programs run in a closed environment, they do not have any more specialty than ordinary apps in generating security holes. Some may ask, if Apple allows loading custom executables, doesn't it open the gate for competitors like Flash/Java?  Truth is iOS has no support for JIT compilation (except Webkit),  apps written in competing technologies are impossible to keep the same performance, therefore Apple always has the upper hand. Having apps like Aemula on iOS is actually good for Apple.

1. Apple has the best developing environment for writing emulators. Good emulators have parts that are written in assembly language. I can write and debug assembly in xcode, but it is impossible to do so on Android. For the Android port, I actually had to write and debug the core on iPad first, then I moved the whole thing to Android. This is a perfect example of how Apple beats Android on native code development.

2. Emulating x86 programs on ARM is very performance demanding, Aemula is a perfect example to show case what Apple's devices are capable of. Now that is all I have to say for all emulator developers/users who love iOS platform. It may not go any where, but if Apple really cares about right and wrong, then it deserves some attention from the people who can make decisions.