And the surplus older 737s are exactly why Boeing has a lock on this sector of the market. There aren't many surplus, older A320s around, so newstarts pick up the Boeing jets. This means they build their maintenance and operations infrastructure around this airframe. When it comes time to upgrade, these carriers -- always cost conscious -- have such a commitment to Boeing equipment and standards, that adding newer models is just a process of upgrading. Even when going to "electronic" controls from the manual/hydraulic, Boeing has designed the process to be as painless as possible.

Kinda like that motto the Catholic Church had: "Give us a child at six, and we will have him for life".

There are very few start up carriers that can raise the capitalization required for new jets. Even if new A320s are cheaper than 737s, a used 737 -- even if it burns more fuel -- is still a bargain. Not everyone can be a JetBlue.