Surely, in the end, it all comes down to infrastructure: enough space for the media and tented villages, hotel and guest-house accommodation for hundreds of thousands within a reasonable range, decent public transport facilities, air-strips for the superstars who fly themselves about, 400-yard practice ranges, the potential to lengthen a course to 7,100 yards or more (pace Lytham), a club willing to allow its ecology to be enormously disturbed by months of heavy traffic, the number of cameras needed for full TV coverage, very private accommodation for Tiger, and so on. We haven't mentioned the merit or potential of the course.
A couple of years ago I had coffee with the secretary of one of the courses often tipped for future Open hosting. He said that they couldn't bear the thought of their wildlife being disturbed by the hordes of media people, spectators and so on. They were not seeking to host it and would refuse it if ever offered. I'll leave you guessing.
You may be surprised at some of the factors involved. My friend at Royal Birkdale was discussing the potential return of the Open there some day. She explained that the school next door did not want to be involved again (despite the fact that they have been on holiday for a week and make a huge profit from the lease of their land for tented village/media park etc.) They were going to have to cut an access road through the sand dunes beyond the practice ground to give the same media access and facilities, but that would be ecologically unacceptable. I have not actually heard that Birkdale is back on the roster.
It's lovely to think that the Open might one day visit the Scilly Isles but it seems to be a matter far beyond merits of the golf course itself.