It took the ZL a while to have any significant amount of development. Bear in mind that the Z1 is very recent and will be several weeks before it is available everywhere. Had to buy mine from Clove because it is going to be at least another month until the US starts to see them.
Since I started developing Android apps at work (been a C# dev for 10 years but novice to Android and Java) I got my own builds of Cyanogen working last week (for the ZL).
Sony does a great job in providing resources for developers. However, it is way less time consuming to develop apps than deal with ROMs... That may explain why you don't see more people doing it ... Some don't want or can invest the time.
Personally, I am happy with the Sony ROMs and would not benefit much for custom. In fact my tablet Z has been stock since day 1. If I was using a bloated Samsung or HTC... Well...
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