Hello! I am not an historian but I am a philosopher specialising in 20th century French philosophy but of course this means I am also well-versed with German philosophers.

Although the Germans did produce a lot of influential philosophers over the period you mention, I am not certain it warrants any exceptional status. France per example had a lot of philosophers in the 18th century and so did England. Same with the 19th century and the 20th century. For every philosopher you mention in Germany, I could probably mention to you someone else just as influential in another powerful nation at the time.

I might be wrong but I have a feeling that your intuition springs from how influential Kant, Hegel, Marx (well, he was born in Prussia but bear with me here) and Nietzsche have been. Although other influential German philosophers were around (Feuerbach, Stirner, etc.) but I think if you look at the historical context, you will have a few answers. The only two contenders to challenge the superiority of Germany philosophically were the United Kingdom and France. France was in turmoil. First there was the French revolution but after that the Napoleonic wars followed by many internal issues. France was simply busy with other matters and was diffusing its culture through the wars it was waging in Europe (Hegel discusses this specifically in his writings, too). Before that, France was probably even more active philosophically than Germany. So this explains 18th and 19th century in France.

Now the UK. In the UK, once again, you had many influential philosophers in the 18th century as well as the 19th century. So I am not too sure where this idea that Germany would be particular comes from. Think of the like of Bishop Berkeley, David Hume (!), Edmund Burke etc.

So this brings us to the 20th century. Once again, many philosophers from the UK, France and Germany can be named, with Germany not being that special in that regard.

But then you seem to claim it ends with WWII. Well, once again, I am not sure that would even be true. Habermas, Horkheimer, Adorno, Hayek (just to name a few) have been very influential. Who can we talk about post-WWII philosophy without the Frankfurt School? So once again, I am not sure it has waned. I have read some papers regarding the loss of influence of German philosophy but that seems to be much more recent and in line with a a shift in how philosophy is done.

Now, I think I should address this perception regarding the influence of German philosophers. After all, Hegel, Kant and Nietzsche seem to be much more central to philosophy than let's say Diderot. But really, it is chain reaction. Hume reacted to Descartes and Kant reacted to Hume. And as the world was changing, Germany had first seats in witnessing the French revolution. A lot of the changes in France at the time prompted Hegel to develop his philosophy of history. English philosophical tradition being what it is, the French revolution also affected the views of the thinkers over there but in a very different way. France was under radical pressure but that in and of itself, if it reduced its philosophical contributions, allowed for a different type of influence in philosophy.

On another note, toward the end of the 19th century / beginning of the 20th century, philosophy started to split between the Analytical tradition (mostly in the UK and in the US) and the continental tradition (Continental Europe; mostly France and Germany). This led to many disciplines in the social sciences to be closer to the Continental tradition as the Analytical tradition engages less with the other sciences as its interests are a lot more circumscribed. Only the older philosophers would be mention in a US or UK curriculum as we are just starting to see an openness to Continental philosophers in the Analytic tradition.

As a last point, it is worth mentioning that around World War II, many German philosophers left Germany or even Europe which certainly could have diluted the pool of intellectuals in Germany (keep in mind a lot of them were Jewish). But even there, I would go so far as to say that WWII probably brought upon Germany a mini philosophical boom while also positioning Germany even more strongly in the continental tradition as those of a more analytic persuasion ended up staying in the UK or the US.

I am on mobile so it's difficult to reread myself! That being said, do not hesitate if you have any questions!