Lot of inbound tourism to Edinburgh, about 3.8 million people a year compared to Glasgow with 800,000. Glasgow and N Lanarkshire have a 25% ish lower average disposable income than Edinburgh I mean don't take my word for it just look at the classic ICAO decode: EGPH: (posh houses) EGPF: (poor f*ckers)
Location is important: Edinburgh airport is to the west of the city and although the traffic is bad can also capture Stirlingshire, Fife and really all the way to Perth and Dundee. To get from any of those to GLA you have to go through Glasgow and out the other side. Indeed if you live in the eastern bits of Glasgow , Edinburgh airport may be easier than Glasgow to get to...
Glasgow still doesn't have a rail link...Edinburgh has a tram (not sure that's an advantage or disadvantage ;-) )