At first, US never ordered the A321 because it was very range limited.
US ordered a bunch after the initial batch of 319/320 aircraft because there was a -200 version that made it able to do the transcons, mainly by adding aux fuel tanks to get the extended range.
The 757 will always have an advantage, because while you have an upgraded engine on the 321, you are still dealing with the same wing, and if you get too radical changing things you lose the benefits of a single type rating for operations. I know when a 321 subbed on SNA-PIT flights, it took a huge performance penalty because rotation speed was something around 160kts.
Similarly, Airbus has an issue with the A318, you have a double shrink of the original airframe, as the A321 is a stretch. Look at Boeing's 767-400 experience, if you get too far from some original design, things don't always work out as intended.
From a passenger standpoint, the 321 isn't bad on the transcon, save from getting stuck in some bumpy stuff around FL310 until you can burn fuel. You may still burn less lower than the 757 due the more efficient engines, though, I am not sure.
I can tell you this, there are many parts, especially in the gear systems, which are not interchangable with the 321's smaller siblings.
