I've never understood why they didn't grab some 1983 stock when they
had the chance. Decades newer than what they've got and the single
leaf doors wouldn't have been a problem on the IOW.

Presumably because at the time, the (at the time, recently
refurbished) stock was still fit for purpose, and replacing it with
the ex-Jubilee stock would have been a false economy. Perhaps if
there were six or seven units of 83 stock available now, then it might
be worthwhile, but with 69 stock becoming available soon, I think
there's a strong possibility that some of them will head to Grockle-
Central, rather than straight to CF Booth's tin-can factory.