I don't think the difficulty in obtaining higher CI is the aircraft, it is the nature (and number) of the destinations.
Marketing spending is related to number of destinations being flown. Each destination increments the amount spent (until some ceiling is hit). So a 1000 pax route is 1 destination, 30 pax route is equally 1 destination. The potential revenue (depending on distance etc.) of the first one may be 33x the second one, but the marketing spending increment is the same.
This means that an airline flying a single 30 pax flight to certain number of destinations would have to spend equal amount on marketing as another airline flying 500-1000 pax to the same number of destinations to get the same CI.
The problem is the percentage of revenue this amount of marketing spending is for these 2 companies. It is going to be much larger percentage for a company flying to number of thin destinations. It is best to not even try to go to very high CI, when flying thin, regional routes.
It is not even that necessary. It seems to be easier to get good LF on short routes than on long routes.