For their own sanity, maybe? Again, other groups managed a better balance of dependable staples while revolving new stuff. I already mentioned Hendrix but Jefferson Airplane is another good example. If you look at their setlists during the same time period (68'-'70) there was way more variation.
Your mention of the blues covers basically underlines the root of my frustration: though he was always somewhat inconsistent, at his best, I consider Townsend to be one of the great songwriters. And like many folks, I also consider the band one of the more talented of the era, in terms of technical ability and strength as a live band. So it's kinda lame that there are no recordings of so many early Townsend classics being played live by the killer group who had originally performed them just because they wanted to jam on "Young Man Blues" and play it safe by keeping their setlist the virtually the same for a few years. Though I do think that learning to perform "Tommy" in its entirety live probably took a bunch of time and was probably what they were also focusing on rather than "Sell Out" material, unfortunately.