Well it's also frequently a function of the band's personel turnover rate. Arsis, for example, seems to change at least one member every time they start a new tour, and I've seen them headline shows where whatever lineup they had at the time only knew how to play like 30 minutes worth of material because of time constraints. Barely enough to fill up a proper headline slot, let alone change it up from gig to gig. To take the other extreme, Katatonia seems to change up their set list on a daily basis while on the road sometimes, which isn't too surprising considering they've had the same lineup for close to a decade.