Thorin in the Blue Mountains

[Kuruharan]

leaving aside Thorin, why wouldn't one of his forebears have thought of doing so

Because I needed a nice clear and concise title to get my point across.

maybe there already was a Dwarven King of the Blue Mountains when Thorin came there. I mean, do we know what happened to the Dwarves of Belegost and Nogrod (the Firebeards and Broadbeams, IIRC) after the drowning of Beleriand? Did they all migrate eastward and merge with the Longbeards of Khazad-dûm

We never hear of a king or two already being there but that doesn't entirely rule out the possibility. I had always had the impression that Belegost and Nogrod were both destroyed in the drowning of Beleriand, but there were still some dwarves there.

as the Chieftains of the Dunedain did not call themselves King or Elrond designate himself "High King of the Noldor" after Gil-galad - there was just too few and nothing that could really be called a realm.

That is a very interesting parallel I had not thought of before.

the major Dwarven settlements we see in the books seem to have been chosen for their proximity to precious metals

Also a distinct possibility.

It probably was some combination of the above...or it could have been Tolkien was trying to tell a story and Thorin being content where he was would have ruined it.