I suspect that some of the differences have to do with culture. In the European garages I've seen, the mechanics have a small set of tools, and the garage supplies whatever specialty tools are required. For example, if you need to burp a coolant system, the garage supplies the specialized tool. Here, the norm is that each tech would have such a tool.
Exceptions are dealers which tend to have more make-specific tools because the manufacturer requires them to have them. When the VW Phaeton came out, dealers that carried them were required to "invest" in over $10K in special, Phaeton-specific tools, and send techs to training on these very complex cars. This is not atypical for any line.
Another reason is that American repair shops tend to be broader in their focus, handling general repairs on every aspect of the car. In Europe, there are actually shops that only do electrical or fuel injection work. The net of that is that while there are more specific tools for the shop specialty, fewer tools overall are needed, as only the tools needed for electrical or fuel injection are needed. Fewer tools, so smaller boxes.
And finally, I think Europeans pay more for their tools, which probably affects sales. It's harder to fill a big box when tools cost so much more. When I saw tool prices in Europe, they seemed to be similar to, or slightly higher than US prices- except they were in Euros, not dollars!
