Only the Star War saga has, I believe, the adequate number of adult AFOL (much bigger than technic in absolute numbers) able to sustain a UCS collection with 4000+ pcs and a retail cost of 300$+, that is the reason probably.
I also would love to have bigger and more complex sets, nevertheless i have to admit that the last truck 8258 for example is already at the par with MOC's around the AFOL world, same goes for all the flagship sets in the respective year...