Why has most 'post-classical' philosophy come from Germany/Austria/France/Britain?

You really to distinguish between Anglo-Saxon philosophy (UK/USA)and Continental philosophy (non-English speakers). This sub-division is part of the observation which led some authors to argue that what "the West" calls "philosophy" is no more than a set of conversational paradigms of limited "use". (Wittgenstein, Rorty). Exceptions (Schopenhauer, Heideggar for example) have attempted to borrow from Eastern tradition, or avoid the Western paradigms completely, but such exceptions have often been dismissed out of hand.

Schopenhauer and Heidegger being a case in point though - both German. I mean, why has it come from these countries rather than Spain, Sweden, etc etc?

I have a feeling that there are several Spanish existentialists and Maturana is a celebrated South American . (Chilean I think).

A point worth considering is that much so-called "philosophy" originates from other disciplines (particularly science and literature). Maturana is such a case.

Isn't the Spaniard, Unamuno, considered an existentialist?

Yes. Lack of recognition is probably a function of translation dynamics.

Greetings
The reason of why there are more is not related to philosophy its related to kind of freedom on those ages. The starting is from britain with john locke (if im not mistakin' with spelling his name )
David hum (**** these spellings) then it went to france so it compeleted in germany and u didnt mention Netherlands with Spinoza so it doesnt matter where they were
Only kind of government matters
And nothing else matters

great thinker,

I mention in passing that "great" in German is translated as gross !

I didnt put stress on that u just went so far
I meant on those ages spain had more dictators than those countries its related not to philosophy

But then you know...gross is great !

philosophy is really just pot talk gone wrong, however absent pot these folks had to make do with hashish, opium, laudanum and absinthe, so it's no wonder they were so prolific, motherfuckers was wasted

I think it's just the arrogance of our ways. We could not recognize the value of anything that didn't come from one of our own. Historically the european philosophers may be important, but the way I see it, all they ever did was try to reinvent ideas that have existed for a long time in other cultures in the world.