They were presumably not coming up with these setlists to appeal to posterity, but to try to get the best concert they could produce for the night. So if you know what the 15 best live songs are in your repertoire, why would it make sense to switch it up? The only reason is if you think you would have a lot of fans who will see you multiple times in a short stretch and want to be rewarded with different songs, but I don't know how realistic that was in those days (and certainly, I imagine they didn't play in New York often enough to worry about that - it would have more likely been an issue in London).
The band clearly enjoyed playing blues and other covers live, more than they liked recording them. So maybe they were consciously trying to give live audiences something they couldn't get on studio albums. Also, since a lot of fans liked the bluesy up-tempo mod approach and might have been not into the pop songs of Sell Out, keeping their live shows with a more "back-to-basics" approach might have been a conscious choice to keep appealing to the longtime fans.
Bear in mind too that the timeframe for all this is much shorter than it is today. If you're expected to record a new album once a year or more and also play a lot of live shows, the idea of adding time to learn a live arrangement might not appeal.