Because seniority as the only metric, that early in a Pilots career, is more harmful than good. It also effectively locks out external Pilots from the joining the Jet Fleet and pushes more in favor of the Companies proposed Cadetship which is nothing more than a B Scale in disguise.
Interviewing Pilots for Jet Jobs based on experience is a much fairer system, an equalizer if you will, where Pilots who didn't fall into the "slot" from Day 1 are allowed the opportunity to Catch up. If you were lucky enough to Join the Links with 500 hours, you should expect to be there longer than someone who joins with 3000 hours.