The reason is quite simple: an expressway in from the north was to cross the A80 here, turning into a motorway (what is now A73) as it passed through. The flat roundabout was just a temporary measure. The diagram above shows a three-level stacked roundabout, which appeared here in the 1967 plan, though diagrams have been found showing what appears to be a much larger free-flowing interchange. This may explain the large surplus of land around the junction.
Of course, the examples of unfinished business detailed above are just a handful of the clues the two highways plans left strewn across Glasgow. If you take a look at the plan using the overlay map, zoom in and look carefully along the line of many of the roads and you'll find the evidence quite easily.
The route of the North Link Motorway is a good start, with areas of empty land almost all the way along its line - the only substantial blockades are housing built in the last two decades. Likewise, the housing developments south-west of Paisley, around the banks of the reservoir, were obviously designed with the motorway in mind.
Everywhere, across the city, you'll see built-up areas that end in a curve that conveniently matches that of a non-existant sliproad, trees cleared for no apparent purpose along a motorway alignment, and so on. Half the fun of Glasgow's unfinished roads is just in knowing where to look.