> Why do you think that might be?
This kind of rethorical question is annoying. If you have an opinion, state it.
> Perhaps the Germans could make their IPOs more business friendly
The main reason companies tend to get listed in the US (or, in the case of many large existing companies, to get listed there on top of their existing listing in their home country) is that the US stock market is the largest in the world, and listing there means easier access to more would-be buyers, and therefore better market capitalization.
I suspect your advice to German policymakers wasn't "somehow make Frankfurt the largest trading place in the world", but that's litterally what it would take.