If I would guess, it's a money thing, I also think it kinda is due to the way the 900s in particular have been redeployed since the arrival of the 220s

Gone are the days of the 4h30min IAH-YYC. The 900 use to be the King of long thin routes, but you're seeing them a lot more on 1hr flights in the American NE for example.

That's nothing official, just my brain wondering