It is simply because they do not want to.
Or more accurately, because they do not believe that they can turn a viable profit doing so.
Why that might be is purely speculation. It is very realistic that licensing is a major factor and possibly the decisive factor, but we are not ever likely to know for sure. No company makes public its decision process on this sort of thing. It is not meant to be understood, even by an enthusiastic consumer base.
However you can be very sure that Tamiya believes that it does not make sound financial sense to put out new automotive kits. And until they believe otherwise, we simply will not see any more car kits from them.
Besides, cars from 10 or more years ago are just much cooler than what's being made now.
