Because its too unusual to actually get the right feel,
Short frames with high BBs are designed for competition riding, where when on the rear wheel it feels more comfortable, also, technical lines where you find yourself upright on the bike allow you so stand straight on the bike instead of hanging off the back..
However, higher BB bikes throw your centre of balance higher, making it more difficult to stay balanced on two wheels, and given two wheel moves are what beginners feel more comfortable and allow them to progress to a higher level quickly, where they can then feel the benefit from moving to a different bike...
My current bike has a +40 BB (stock)
And is the highest BB I've ever had,
I don't find myself in many positions where I'll need to be very upright on the bike so I don't need it..
High BB doesn't always mean its good, the only reason the high end bikes have high BBs is because they're competition bikes..