Because in some airports, a priority tagged bag seemingly already means last on carousel. Having a First Priority? That's just about right on cue for handlers in those airports to just send it off to Havana or somewhere suitably exotic.

More seriously, as OFFlyer said, I would be more concerned about getting the elementary things right, i.e. actually delivering the orange tagged bags before rather than after all the other bags. Once that's sorted, maybe they can try the two-tier priority thing, but I don't fancy their chances too much.

If you want to see military-style accuracy in "ranks" of bags being sent down the carousel, look no further than SQ where on every flight's belt, the bags will always come out in the First Class -> Raffles -> Economy order. Always.