There is always the Do328 that sits in Dundee for most of the day between Stansted runs. No reason it couldn't position to Glasgow and operated GLA-IOM-GLA as a daily service, opposed to Citywings double daily.
There is a very good reason. People have proven on here they are unwilling to pay £99 to fly off the island (I'm not convinced how many would be genuine customers anyway). They've also complained that the schedule is wrong, aircraft is wrong, and frequencies are wrong.
If people are unwilling to pay £99 when the aircraft has no dead legs, how much do you think the price would be if the aircraft was running DND-GLA empty to operate a GLA-IOM? Closer £200 is my bet, certainly not £20 that everyone seems to unrealistically want