I would guess it's some combination of wanting to keep the cast salaries lower and wanting to make sure that the cast they got wouldn't have a ton of scheduling conflicts as they'd be required to film five seasons in New Zealand. It's also possible that Amazon Prime, as a newer studio, has a tougher time getting big-name actors to join their shows than established studios like WB and Disney do without paying out the nose for it.

Will Poulter was originally supposed to be in the show and had to drop out because of scheduling conflicts. Guessing any known commodity would have had the same issue.

FWIW I think the cast is solid and not at all a problem for the show.