Glasgow and Edinburgh have pretty similar sized catchments, but Edinburgh has a much greater number of inbound tourists and if you have a flight as unique as SFO then I also think people will come from as far as Glasgow to use it. Its hardly a ten a penny flight to a med destination. I don't think you can compare the Edinburgh end of the M8 or the likes of the city bypass to Glasgow's urban motorway either and road is the only real means of access to GLA. I only live in Bishopbriggs and it can take me as much as 45 minutes during peak times to get from the m8/80 merge to the Kingston Bridge.
I mean the likes of CPH I can see a place for in Glasgow when there are 4 operators at Edinburgh, because not all of the 100k+ people using that service are going to Edinburgh. Surely though if these services were viable, they wouldn't have been dropped or they'd be being run again already?