I think that the big problem behind the dropping of the EDI route is likely the timing, especially during the week. I used the flight last week on Thursday & Friday. Arriving down to SEN at 22:30+ is not an attractive option for most people. Getting up to EDI at 20:30 is just about OK, but is still on the late side.
A service like this needs to be timed to let the users get to where they are staying, usually within about an hour's travel from the airport, and still be in time to have dinner at a restaurant, otherwise it will be self limiting.
Earlier last year, the timing was good with a mid afternoon slot which gave you the option to work in the morning, travel, have a relaxing or productive evening and then have a full day the next day.
However, the current timing is just self defeating. The flight down on the Thursday was at best 1/3 full. The Friday return flight was probably about 90% full, mist of whom seemed to be heading out on a night out in Edinburgh (hen, stag etc...). I can't see the return flight on the Friday having been particularly full.....