as kranzy has said, yes, a lot of people have moved on and the cars are getting old now, but there has also been a shift in focus.
the project cars section is quite busy most of the time.. the bulk of the technical areas have everything you would ever want to know about a u12 or a u13 which is why there is less activity there now. but if there is anything you can't find or want to know, there is nothing stopping you from posting up a question and asking. there are quite a few knowledgeable people that still frequent the forums and they still contribute even though they have moved on from the u12/u13.