@theogrin I think the original reasoning was that there was a limited amount of vaccine and they wanted to apply it where it would do the most amount of immediate good*. But now they have loosened things up (due to, I assume, more vaccine) and this framing is still in place, but having been expanded the framing seems less meaningful.
* This, of course, raising the question of "what if you've made a miscalculation about the method of spread"