The clowns usually have to acclimate slowly so that their slime coat can build up. Even clowns and other damsels (same fish with different costumes) can get captured and eaten if they just dive in without making changes to their bodies first. This is likely why it took a while. You can sometimes see the clowns dive bombing for a few weeks until they decide to move in full time. There are stories online of people forcing clowns into their Haddoni too quickly in small acclimation boxes/tanks and getting captured and eaten.
I had a trio of Semilarvatus once that could hang nearly in a Haddoni sometimes without getting eaten. I have no idea why, but when taking with Fenner about it once, he wondered if they could also develop more slime since Haddoni were native to the Red Sea too.