It's the route structure and how they use pilots.
right now if a plane if pilot goes down its plug and play. With another type they couldnt substitute as effectively.
The only way another type works is if that plane does something very specific, ie long haul.
You couldnt put 100 Airbus into the network it wouldn't make sense, dollars.
50 220s island hoping? Maybe but that's a big cost for something different.
they'll wait on the max 7. They are pot committed