Not only do we have line bidding with unparalleled transition language, but we also won (after many years) the whole red/green debacle. Voting in PBS would nullify all of that and usher in a whole new era of grievances and arbitrations.
Also note that the latest FA TA there was a note that not only would there not be any PBS for them but no obligation to even discuss it in the future. I think this would seem to indicate that the company has resigned itself to line bidding as they would want to spread the cost of PBS implementation over both the pilots and FA's.