From a cursory glance at the Boeing performance charts for a 738, 5,700 feet of runway is way more than enough when dry but at certain weights, not quite enough when wet.

To answer the OP's question, there is no difference between the early-delivered 738s (1999-02) and the more recent (2009-current) 738s in the area of landing field requirements.

Add in wind speed and direction, other pilots' reports of braking effectiveness, whether the AA plane was light or rather heavy, and I can't second-guess the captain. No pilot wants to command the next AA flight to run out of runway on a rainy night (like Kingston or Little Rock, among others). Safety first.