Keep in mind that AA was primarily an MD-80 fleet in the early 2000's, so the 737-800 was a 'big' plane. They also had 757s for the higher capacity routes. AA also went years without an narrow body aircraft order after the 737-800s of the late 90s since they inherited the TWA fleet.

When they did decide to make a narrowbody order to start replacing the MD80s and 757s, the wisely split the order between Airbus and Boeing, with the 319 being a partial md80 replacement and the 321 being the 757 replacement, along with more 737-800s to replace the MD80s and help growth.

I have flow the 737-900 on Delta and it is a miserable plane. I much prefer the extra room of the 321.