[iggldiggl]

France operates on a "periphery-to-Paris-and-back" model, so the goal is quite clear – you build a radial line towards Paris and voilà, you've sped up that relation. "periphery-to-periphery" traffic is mostly ignored, so you get the somewhat curious situation where the geographically direct connection between two cities might be slower (or only marginally faster) than going all the way into Paris and back out again.

Germany on the other hand doesn't have a single city that's as extremely dominant as Paris, so to capture a similar amount of traffic on your high speed network you need something more like a mesh, where any single route might only capture a smaller fraction of the total traffic, rather than the simply radial star you can get by in France.

Of course that's not the only reason for the differing evolution of high speed rail between France and Germany, but it definitively is one of the reasons behind that difference.