[Someone]

Forcing the use of a single rendering engines also minimizes memory usage (the code for the engine can be mapped in shared memory) and thus allows Apple to ship devices with less RAM, slightly decreasing power usage. It also speeds up the launching of applications that use a renderer (the code for the browser, its JavaScript engine, the font renderer, etc. will already be in memory)

I think that effect could be fairly large if Apple would allow applications to contain their own web engine, not because it would mean a second engine, but because it easily could mean a third, fourth, etc. engine, given that not all applications get updates at the same time. Imagine app #1 shipping with Firefox's webview version X, app #2 Firefox version X with bug fix Y, app #3 with Firefox version Y, etc.

[pcwalton]

Apple allows Unity and Unreal Engine, all of which are just as big if not bigger than WebKit. Leaving the realm of games, they also allow Mono/Xamarin, which is a similarly enormous framework.

The restriction is plainly a business decision, not a technical one.