And to answer your question:
The UCI's Tier system is basically based on the amount of money that the team's sponsors are pouring into the team for bikes, travel, salaries, personnel, etc. Your division is based on this, and generally speaking, the biggest and best funded teams can afford to pay for the best riders, so you get a sort of natural selection that way. The best stay the best as a result of their sponsors forking over the $$$.