The answer is very simple - ATA operate their L15s in a high density configuration, and the FAA has a requirement that exits must be no more than x number of feet away from y number of passengers. As ATA is the only US charter operator of the L15s, it's the only company to have modified their aircraft, using the aft doors off longbody L1011s. They are designed for one passenger to exit at a time; whereas the wide ones are designed for two pax to exit at a time.
All of ATA's -500s came from Royal Jordanian.
Incidentally, only British Airways had the double sized doors in the aft position on their longbodied L1011s - that's why the Caledonian aircraft are currently equipped to carry 393 pax, versus around 350 pax for 'normal' longbodies.