Siegor: Was he using puns? Those usually only work in the native language and are very hard, if not impossible, to translate accurately.
I'd say it must be a combination of this, and also the use of 'common places' that you don't get if you aren't German or aren't familiar with the German culture (e.g. he mentions a particular brand of mortadella with a smiling face that I guess must be rather popular in Germany). But on the other hand we've had many German speakers in this thread who didn't think this guy was particularly funny, or funny at all, so I don't know...
Now, forgive me for going slightly off-topic, but I'd like to contribute something that *I do find* extremely funny, just as a little experiment to see how much can be lost in translation (and subtitles). Here's the only video I could find with English subtitles of the (IMO) best comedian in Spain by a wide margin:
https://www.youtube.com/watch?v=ImGfd-pNoXc Pity that it's only 3 minutes long and it only contains one of the 'ideas' or 'concepts' that make up his shows.