It's Marketing that really kills the small jets flying to smaller destinations. Since your Marketing budget is a factor of your routes, and smaller regional airlines tend to operate to many more, and narrower, routes, an all-regional operation can find themselves putting as much as 40%+ of their total revenue into Marketing just to maintain a good CI.
In this latest Modern game, I was flying a fleet of 16 x A300s. I then decided to use Fokker 100s to get to smaller destinations. Yesterday I put 3 of them into service and my CI Marketing budget went up 50% -- a huge $3,000,000/week increase. That's 5 TIMES the money that those planes could even hope to bring in. My profit disappeared literally overnight. But since the system calculates your Marketing budget based on the number of routes you have, and putting those planes into service flying to many smaller destinations increased my destinations significantly, I'm screwed. Every single route I open up with those planes costs me so much more money with the incremental increase to my CI marketing, that I actually LOSE money (a lot of money) on every route I fly to.
Here's my chart. You can see where I hit the thresholds for CI Marketing increases throughout the year, then, 3 weeks ago, you can see where I put those 3 Fokkers into service and the HUGE Marketing hit that I took as a result. The bump in there about 3 months previously is where I put 3 A300s into service -- the marketing increase was TINY compared to the cost of putting 3 Fokkers into service since they went to more destinations, and those A300s make a LOT more money too.