Actually, I wouldn't put it above Alaska Airlines and Southwest Airlines to both place substantial orders for the 737-900ER. That means Alaska Airlines can fly more passengers on longer USA transcon routes and Southwest can use them on nonstops between USA West Coast cites (Seattle, Oakland, San Jose, Sacramento, Los Angeles, Las Vegas and Phoenix) US East Coast cities (Providence, Islip, Philadelphia, Baltimore, Jacksonville, Orlando, West Palm Beach and Fort Lauderdale).
Perhaps Southwest wants to be able to compete against JetBlue for the transcon market. The 737-900ER should have advantages for Southwest compared to JetBlue's A320's. The 737-900ER carries more passengers and will be able to perform transcon service year round without technical stops to refuel. JetBlue sometimes has to refuel on their transcon flights. The 737-900ER should also have better performance at hot and high airports like LAX, Phoenix, Denver, etc. than the A320.