What makes you say the terminal is a dump? It has more space and is less claustrophic than EDI IMHO.
We can blame AGS til the cows come home but for me it's all about location. There was only ever, ever room for one major Scottish lowland airport. BAA made a strategic decision to support GLA at the expense of both PIK and EDI, (Joe Curry was right, I have changed my mind!). Once the focus of politics and economics moved East and EDI and GLA were both sold, the free market itself coalesced around EDI as it had a relatively stronger inbound market. That's not to say GLA can't make money, but when American and United have data for both, they've both swung East, Air Canada have data for all three and settled on EDI. That's a natural behaviour reflecting relative demand. Now AGS can discount all they like, but I suspect the airlines will continue to pay a premium to use EDI, who can offer a good introductory deal and show value in remaining with a relatively better performing route. GLA was my home airport once, but like PIK before it, the future isn't always the one you expect. They can do well and make good money, but I suspect always in the shadow of the capital now.