Hey Bubba,
The original draft included wording that allowed for the possibility of grandfathering the DM-One spec (only in its current form) to allow it to run until they decide they're going to make a new model, or just run with the car that's already legal. The language about what items to grandfather in spite of the rules didn't make it on to the forum, but it's still an option none the less.
In the interests of making it much easier for other manufacturers to become involved, the rules have to go one way or the other, and in deference to the fact that it costs nothing for other manufacturers to use a chassis that they're already making, this seems the logical choice. Hong Nor and Hobao both make buggies and racing trucks that would fit right in, and the GTP is currently legal.
In it's current form, I don't think it's an advantage they way they've constructed the car, but to leave the door open for other manufacturers to pull it off successfully would also allow for the possibility that all others would have to follow, which increases the cost again. The rules are written to keep it entirely based on the chassis from a current buggy/truck, which is the way it's been for 20 years. It's intended to keep a level and inexpensive playing field.