To Peter's question (about Wales), a whole gamut of reasons, but I think principally no course/facility of sufficient golfing challenge that is (at present) quite big enough for the Open, local and regional transport infrastructure, and suitable accommodation. Ryder Cup Wales at Celtic Manor was a long-term Welsh Assembly-supported project, and local and national government has also helped bring the Curtis Cup to Conwy this coming August (it will be the third Welsh venue for the CC, after Porthcawl and St Pierre). Not coincidentally Conwy is only about 80 minutes' drive from Manchester Airport.
NB that Porthcawl (which has already hosted two Senior Opens) will host the Ladies Open in 2025. RStD Harlech has happily hosted the Girls' and the Senior Ladies' in recent years, and but for COVID would have hosted the ladies' Home Internationals this past year, but the economically depressed Harlech area badly needs a new hotel.
(should perhaps add in the interests of 'full disclosure' that I am a longstanding individual member of Golf Wales, formerly the Welsh Golfing Union, and hence not a neutral observer in any of this! Hope this response provides some answers anyway.)