The A321's are 12 years old. The 4 eldest A320's are 9/10 years old and the rest are from 2004 on. So maintenance costs are much higher and tech delays far more common.
The A321 burns about 15% more fuel than the A320 so if the A320 is full, you need 200 on the A321 to start justifying the extra fuel burn alone. It's a common day to fill 174 seats but a whole lot rarer to fill 200 plus, and that's in the summer. In the winter, it just doesn't happen.
If it's more profitable to fly 174 pax in a 320 than 199 in a 321, then bye bye 321. Simple as that.