Why? Because if you ever want to upgrade or expand to aircraft with 200+ pax & 2800 nm you can get A321 and stay fleet common. Not bad with fuel either.
You are going to get a lot of responses about MD80, 737NG.... please disregard those. A320 series has a proven track record (at least in AWS) to be the most versatile. You can serve a 200NM route with 318 with high frequency or you can kill a A333 with A321 tech stop.
But then again.... you should go for MD80 since its very popular amid airlines waiting in line to BK
Sorry, but with fuel's usual spike in MT scenarios, any plane carrying 100 pax on 2000nm+ routes will lose money. You might marginally "make" money if you own the plane, though, its more accounting wizardry than it is actual profit...
On a per seat basis, the 318, 319, 733, 736, 737, M87, etc are simply not competitive compared to the larger planes in the series. I would not consider flying a 2000nm route after the year 2000 with less than 150 demand (unless it was a competitive or boredom situation).