Every time a beta release for Minecraft Pocket Edition is released, countless iPhone users ask why they don't get the beta builds. There are two primary reasons for a lack of PE betas on iOS, the first being it's incredibly difficult, and the other is because it's unnecessary. It's difficult because among other requirements, Apple requires the developers running the beta test to manually enter every Device ID that will receive the beta. Mojang developers don't have the time to enter thousands of Device IDs into their beta program. It's unnecessary because testing applications on iOS requires only testing it on the few iPhones, iPads, and iPods that can run it. By comparison, there are thousands of Android devices that could have bugs with MCPE.
Yesterday Apple bought the company Burstly, which is known for creating tools to make beta testing on iOS easier. Their main product, TestFlight, allows developers to easily install their apps on beta testers' devices without first putting them in the App Store (therefore bypassing Apple's less-than-ideal beta testing method). It's possible that Apple may use Burstly's TestFlight technology in their own developer tools in the future, possibly giving developers a new way to beta test their applications on a massive scale. If something like that happens, perhaps Mojang could utilize it to create iOS Minecraft Pocket Edition betas. But if they did create the betas for iOS, it would be for the player's benefit and not much to theirs.
Keep in mind that even if Apple does eventually integrate TestFlight into their own Developer Tools, it would still be up to Mojang to test and develop iOS betas.
Source: The Verge