From this interesting article on dive bombers:
http://www.australianflying.com.au/n...e-bomber-myths
"By the time the Trumpets of Jericho (Jericho-Trompeten - dive sirens) were being discussed in the Allied press, they had had their heyday and were being phased out due to their
lack of surprise effect and because of drag."
The Ju-87 probably lost its air brakes for the same reason, to reduce drag. The faster you dive, the faster you come out of the dive, making you less of a target.