This is a big reason.
Smaller expedition vessels have a huge advantage over the larger vessels. Quite frankly the smaller vessels (with shallow drafts) can get much closer to the Kimberley highlights than the larger vessels. .
This gives passengers access to areas that larger cruise ships can’t reach, allowing for a more intimate and authentic experience of the Kimberley’s natural beauty. And it means you have much more time ashore to explore the Kimberley – take longer walks, find refreshing (and safe) swimming locations and so on.
Sure all the big vessels have zodiacs but the trip is longer in a zodiac when your vessel has to moor a long way out. Consequently you get less time at the location.
(And there is a limit to how far you can go on zodiacs open to the elements to be frank. That Kimberley sun can burn you quickly).