The chief reason (at least in the US) is probably due to standardized testing having a white-list of allowed calculators. Your phone can emulate the TI-89 just fine (and run other tools like LAPACK, PARI, Octave, etc.) but good luck getting it approved for a test.

Just goes to show that the US government is not the only organization capable of crippling progress.

I was chatting with my dad about this today. (He teaches classes that use graphing calculators.). It's cheap to buy a calculator emulator app so they must allow it soon. Though students sometimes use the camera and email to cheat.