That's a good question.
The last time I flew HA PHX-HNL, there had to be no fewer than 25 wheelchair bound people lined up to preboard.
The key to an evac is speed. The people easiest to evac are the ones who can do it under their own power. Generally, they are the quickest to be able to evac too, although the current trend of wanting to get all of ones carryon bags first before evac-ing has shot that theory down somewhat. Still, the priority is for people to evac quickly, and the able bodied are the easiest to.
The problem with ground evac’s is that there is generally no help from CFR in doing so, if the aircraft is sitting on all its landing gear and only the slides and cockpit ropes are being used. If the plane is sitting on its belly, then assistance is available. But an aircraft on its landing gear, the slides are one way, with the only assistance being at the bottom. Even if firefighters ladder-up to the wing, they can’t get inside the cabin from the overwing hatches, as that will only hinder, slow, or stop the evac, putting many pax at more risk than already exists. The bulk of people have to get out first with the evac trickling to the final random pax, before entry can be made to assist pax that aren’t able to evac themselves.
The good thing is that time-wise, if an emergency allows for all exit slides and overwing exits to be used on both sides of the aircraft, then the evac of the bulk of pax will go pretty quick, allowing for the rest of the pax who need assistance, to be able to get it from firefighters pretty quickly. If, however, only certain exits are able to be used due to fire, fuselage damage, etc, that just makes the situation worse for not only the able-bodied pax, but especially so for the ones who need assistance.
It sucks, but the idea is to save the most amount of pax in the least amount of time. Still, every effort will be made to evac everyone, but people have to realize the overall strategy and the reason(s) for it.