Well 2 situations
Dps in question has not read and comprehended the tool tips (happens a lot apparently)
Due to mob movement the dmg gain of landing the positional is less than a retry if the move misses the proper side. Is moved, mainly trash fights. On drg I stun something to make sure it will not move during positionals, but that is not always an option if there is a specific move that needs to be stopped or both stuns are down.