According to the bean counters, the two most profitable aircraft are the 757-300 and the Big Buses.
The -400? No clue. Limited range. I'm making crap up here because I really don't know, but when the company bought the -400's, they were replacements for the domestic L1011's bouncing between high volume markets in Florida. Hell, they weren't even strung for HF or SATCOM at first so you really couldn't do a lot with them besides domestic flying.
Then the company poached a guy from CAL that had the attitude that big planes only make money when they fly far places, which makes sense. The 777's shouldn't be doing ATL MCO turns and ER's shouldn't be doing ATL to JAX. So, again I think, the -400's once they hit the international markets, it just didn't perform as well as the ER's and especially not the Airbii.