I mean, in this case he’s using the mask, but I’d say that it has a couple of reasons. For one, ice might be a strong material, but it isn’t usually thought of as such in media; see the absurd number of weaknesses Ice types have in Pokemon, especially the more predominantly physical types like Rock, Fighting, and Steel. For another… well, ice melts. I guess if you can control it you can slow down the process, but I’d imagine many battles are fought in a place where it’s above water’s melting point, meaning the shield is going to naturally weaken relatively quickly or the person using it has to put effort into making sure that doesn’t happen. Neither of which are optimal, I’d imagine.
Anyways, glad to see that this paid off for Kopaka. Just seeing him step out over the lava last time I was ready for things to go south real fast, but I guess the Hau is pretty useful.