The impression that I got was that Insel are all right and considering previous criticisms, have got their act together to a large extent.
Apparently you didn't read the most recent reports in the Insel complaint thread I linked previously. Just because they were on time for your flight, the one and only time you ever flew with them, doesn't mean their shoddy airline is "fixed".
For the timeframe we can go (November) Insel is $378 roundtrip from Miami, Delta is $1156 roundtrip from Atlanta.
All said and done, Miami and Atlanta are about equidistant. Our local airport is Jacksonville. It seems you really can't get anywhere meaningful from here for less than $400, hence the willingness to drive.
You're almost always going to pay more when departing from an airline's hub city, such as with Delta flights originating in Atlanta. Thus the term "hub captive" to describe travellers who live in hub cities.
You can fly Delta from Jacksonville to Bonaire, connecting in Atlanta, for almost half of the Delta price you mentioned. Heck, you can fly first class from Jacksonville on Delta for less than $1000.
Here's one November itinerary for example: