Because (apart from the days when Suros ruled crucible) the other exotics weren't the answer to all activities. Gjallarhorn is viable for absolutely everything and is undeniably the best and most OP exotic in the game. The exotics being brought forward will be used, but not the only guns being used, whereas gjallarhorn would be the only one getting playtime. Basically, it goes back to the problem that gjallarhorn is too good at what it does