It wioll be down to a variety of reasons including costs. You cannot just start a service, you have to put into position ground services, luggage handlers, check in desk and staff, catering, fuel bowsers, local flight crews etc, etc, even if you use an external company, they would either charge a huge cost as they would have to employ staff for more than just the day of the arrival.departure, you have to be able to position aircraft, if you take an aircraft from the Manchester/gatwick stable, you would have to have a replacement aircraft, plus once a week would probably be totally uneconomical compared to the services and set up at Man & LGW.
You have to negotiate slots in UK airspace and at Orlando airport, or take from already profitable slots at Man or LGW. Where is the incentive for Virgin? Why has no other airline done a regular weekly flight? If the demand was there then there would be a few airlines wanting to run that route.
With the flights you already have, it must cover demand. If flights are being run from another airport into Glasgow, its because there is not enough demand for a single service unfortunatly