I'm not an expert, and I don't work for the ECNL, but I'll take a crack at Florida.
Many Florida clubs, especially in South FL (Dade, Broward, Palm Beach) turn down R3PL bids because of the distance and cost of travel. Also, as you state, there are so many good teams at each age in South Florida, they feel they don't need to travel all over the southeast in order to get good competition.
I'm not 100% sure this is the reason, but I did ask a similar question to a Ft. Lauderdale soccer parent and that was their response.