There's White Castle in the SOuth, and also in the Midwest.
BUt the answer to your question is that there's an inverse relationship between quality and how widespread the burger chain is.
Since the US is fair large country, so regionalized chains spring up all the time, their marketing is geared towards offering a tastier alternative to the Big 3 (BK,McD's and Wendy's). However, it the logistics of operating on the other side of the country are complicated since they often use local suppliers. As local burger chains become established it is generally difficult for other chains from other parts of the country to penetrate their market, as they offer similar quality but lack local goodwill.