The modern Olympics are totally political. Athletes can only compete if they are representing a country, and they are subject to direction from that country.

The ancient Olympics were apolitical, and each athlete represented just himself. Some honour may have accrued to the home district of a successful athlete, but this was incidental.

I'm trying to discover whether this fundamental (and to my mind undesirable) change was by design, and if so whose, or whether it "just happened". Indeed, whether it actually has any basis other than custom.

Anyone any (informed) ideas?

There is a lot more to it than just patriotism. It now comes down to money.

Did you know that the origin of "amateur" athletics were the noble class (rich) who didn't want to compete in sports with the stronger "professional" (poor) working class.