Well, for one thing, you have no file system - and that's just first among the list of POSIX-y things that Apple internally has access to, but you do not as a 3rd-party.
The amount of plumbing to be ripped out is pretty extreme given that you don't have access to large bits of the OS.
Not to mention Torque is not written with OpenGL ES in mind - so you'd have to rewrite not only plumbing, but some core graphics code.
To make it an Apple-compliant app, porting Torque (or most desktop engines for that matter) is simply not feasible/worthwhile.
