Yes, a GmbH is a corporation and mostly dissimilar to an LLC. In Germany, there's no such thing as an LLC, except for some professions. So if you want limited liability, you're only option is to incorporate.
Mostly, it’s that Delaware’s corporate law is more developed so investors have more certainty of what they are getting; this is also a self-reinforcing condition, since the preference for Delaware corps means more corporate issues get resolved under Delaware law.