@Scholmes One assumption I have is that Cartoon Network, for some reason, thinks that kids won't enjoy the series or relate to Ben if he's an adult, even though kids like superheroes like Superman, Captain America, Wonder Woman, etc. There's also pointing out that a Ben 10,000 series could also involve Ben's son, Ken, which could make keeping Ben a kid feel even more unnecessary.

There's also pointing out that the Original Series continued to be the most popular and successful show in the franchise and Cartoon Network was trying to recapture that popularity and success, even to the point of making an unnecessary rehash of the original show.