The NBA probably rejected it because they don't want to see contestants get nineteen tries at an insane height, only to have the guys just lower it back to ten and need nineteen more tries to pull off a dunk.
"My hump, my hump my hump my hump!
my hump my hump my hump, my 15th try on this dunk!
Check it out!"
Also, the dunk contest just plain isn't that exciting anymore. We've seen so much of the stuff.