The lead time for booking a visiting choir at a Cathedral gets ever longer. I have been sorting out a week for August 2018 and have booked one of a couple of available weeks in Winchester. A couple of other major ancient Cathedrals had just one week left. But Hereford is booked up and we have to wait for 2019 to try to go there. (After an attempt to book them in 2016 we were told we would be contacted when 2017 booking opened, but we never were).
Part of the reason is that the 3 Choirs Festival will be held in Hereford in 2018, which reduces available time for visiting choirs by a week, but it always seems to book up earlier than other comparable Cathedrals. The Willis organ was in great demand by organists after a rebuild, but that was a decade ago. Any other suggestions why Hereford is more popular than, say, Salisbury or Durham? Is it the cider?
[August 2015: I have a partial answer. They like inviting back choirs whose visits have gone well. One such choir (which I have sung with) sang there 12 years ago and has been offered two summer weeks since then, without needing to request them. Once these choirs have been accommodated, there are few weeks left over for anyone else.]