Did they lack the necessary transport ability to land troops behind the German trenches? Was the Hochseeflotte still too much of a threat even after Jutland? Did they not have enough manpower to spare? Or was there some other reason?
The first two. The High Seas fleet was never neutralised - and while it was never capable of breaking out into the North Atlantic, it was quite capable of intercepting any British flotilla approaching the German Atlantic coast. The Admiralty didn't want to fight a decisive engagement so far from their own support bases and so close to the Germans', so the idea of an amphibious landing - much prized by armchair naval strategists before the war - was never viable.
There is some more information in this thread.