Allowing other browser engines would also mean there'd be a lot more room for vulnerabilities that Apple has no control over. That's not to say that Blink or Gecko or whatever aren't secure, but if either of these engines introduced any kind of severe holes the consequences could be disastrous (and worse, unpatchable for that segment of folks who disable auto update).

Besides that, both Firefox and Chrome on OS X are notorious for power usage issues. There's a very good likelyhood that'd be true under iOS as well and the last thing Apple wants is a bunch of users blaming their iDevices for poor battery performance when the issue is actually web engines that are bad at rationing energy.