One problem is that they have to charge a premium guest price for it which would put some managers and punters off. There are some Spoons around that have Hophead as a semi permanent guest or rotate it with Pale Rider or Jaipur which are also permanent fixtures.
Did you know that the Hamilton Hall does 500ml bottles of Redemption Big Chief for £3.65? This is much better value than the guest ales.
Sometimes you get a dickhead of an area manager sticking their oar in and telling managers to stock only guest ales that are cheap. I believe that may be happening in my area at the moment. The Leytonstone one recently had Westerham guests in rather than ELB or Hopstuff. I rarely go to local JDWs at the moment.