I think it was because the whole code for promotions would need to be rewritten from scratch to fit the Bishop underpromotion on it, and that's time Vas preferred to use coding something else.
Also I think it's the same reason some important endgame knowledge of the engine is missing (so Rybka shows +2.00 eval in some dead drawn endgames), it would take time to implement but not be as effective as using that time to implement something else that boosts elo more significantly.
And finally, Vas has admitted to being lazy when it comes to coding some stuff.
