That, and Glasgow to Edinburgh is something more than an outer suburban route in terms of perceived importance, with quite a lot of 'inter-city' traffic - though certainly not of a length that justifies the First Class bells and whistles of a 'proper' inter-city service. The routes via Cumbernauld, Bathgate, and Shotts are very much Edinburgh suburban and Glasgow suburban services that happen to meet in the middle, and are treated accordingly.
Getting rid of First Class on the Edinburgh-Falkirk High-Glasgow route would probably be poorly received in both cities as being tantamount to reducing each to a suburb of the other, even if it is just a different colour of seat, more chance of sitting in it, and a nice slidey door to keep the riff-raff's noise down.