Emissions I'd imagine.
By the end of production it was a legacy unit that wasn't offered in any other model, it could have been eliminated to reduce the number of different engines produced and give over capacity to other units used in multiple models. But when you consider that it was replaced by a detuned special 2.5 that (as far as I can tell) has only been offered in the Z4, that theory doesn't hold as much water.
I'm actually surprised that they bothered with the low-power 2.5 and soldiered on with the low-power 2.0i, when the 2.0-litre mill in the 1-Series knocks out 170bhp and would nicely replace the 2.2i on performance grounds and 2.0i on efficiency.