Because we have been fighting since the red war ended. Our guardians training is now fieldwork that we have done in both dlc and foresaken. Unless we are gonna start killing other guardians in the wild, I see no point for crucible to still be around in the destiny universe. It would be like a football player training for football and then start playing rugby. Close but not training for the right sport. Shooting guardians in the crucible and then fighting a wish grant dragon does train you to shoot but man theres is more than just pointing and shoot now. Especially now with foresaken, a city rebuilding and learning that just shooting a ghost can kill it, probably not a good idea to have floating ghosts just waiting to be shot by a rogue guardian at any moment. Could turn into another thorn perma-killing a titan agian in crucible.