I would guess because simply historically, we always were very private people. This was generally increased with the experiences during the second world war and after in East Germany, where there was widespread government spying and people spying on each other. Thus, we have lots of groups that are engaged with activism regarding privacy, and it's present in the collective consciousness.
Reddit is an advertising shithole and Lemmy is the more private/free alternative, so it's more likely more Germans come here.
Also in general, the internet lends itself for less social people, which I'd also characterize Germans to have a tendency for.

