I believe the restriction is 48 hours after the end of the National Championship. Tier 1 then typically goes first as soon as they are allowed to and then Tier 2 goes the following week in AAHA. The question isn't why the Atlantic goes early, they don't. The question is why the Potomac Valley goes late.
There is some crossover for kids who live in extreme Southern PA or Delaware. Places like Southwestern Chester County, York, and obviously Northern Delaware are close enough to go down there and there are some that do. It doesn't impact many players or teams but it does impact some. The organization it really impacts would probably be Tomorrow's Ice. They play in the AHF Central now and all of the other teams in that league/division are allowed to eval 2 weeks earlier than they are (Ducks and Team Philly, in particular, compete for a couple of the same kids as TI does). It's a disadvantage for any kids who might want to cross over and play down there when they get to tryout age rather than evals. If you tryout for an Atlantic team and they offer you a AA spot, they aren't going to want to wait 2 weeks for you to decide if you want it while you are waiting for TI to be allowed to hold tryouts. The musical chair game is, for the most part, over by that point.
If it were me and I ran TI that would be something I would try to find a way to work around with PVAHA, if only for players who reside in the Atlantic, and the Selects teams (because those are the only ones impacted by the disparity in tryout dates). Either that or the PVAHA needs to change their dates.