Just think your limiting the routes you can operate by having a bigger plane. Makes sense to have one type but why not a small plane? You can operate far more routes and if you need extra capacity just add more rotations. More frequency is only going to be welcomed by customers
In some cases that may be appropiate, but in other cases not. Airport slots, landing fees etc need to considered. Crewing costs and availability are another factor
Unfortuantely the site is currently undergoing maintenance, but there was quote on anna.aero recently about Norweigan being prepared to sacrifice some routes as it reties it's B737-300's, replacing them with larger B737-800's.