Why should there be a rule applicable to all OPCs? Each one is an individual business, even if it's within a dealership group, with its own targets and customer base. I get the frustrations people have with trying to get certain cars, and the hoops you have to jump through, but unless demand drops substantially then I can't see anything changing any time soon. OPCs can sell any GT car they get allocated several times over, so they have reason to change whatever they're currently doing.
It's not just about loyalty, there are other factors - financial reasons, personal relationships, spreading the cars across as many people and types of people as possible etc. Like it or not, it's a selective process and there aren't enough cars for everyone.
Your suggested appropach, for the GT4RS for example, would cause absolute carnage with a barrage of emails, phone calls and people turning up at the dealership!