I don't know if there is much to understand. Frank is the only one of the original VA's who is still working, and at 74 that he couldn't possibly pull off the Fred voice he originated (It's probably why he's voicing Scoob). No one could imitate Casey Kasem, not the least because he was iconic in his own right before and after Shaggy. Furthermore, if you look at the IMDBs, you'll notice that there isn't a lot of continuity, outside of Frank, Casey and Don Messick; the girls tend to be substituted more than the three male leads.
I think what's more troublesome is that they seem to be condensing aspects of several HB "classics" into one movie, along with an almost assuredly ill-fated attempt to retell the Scooby Doo origin. A Pup Named Scooby Doo is still one of the, if not the most, liked entry in the franchise.