Because Catalogue of Life is the taxon authority for animal subphyla and includes Crustacea but not Pisces.
It’s worth noting that from my understanding paraphyly of Crustacea is a much more recent discovery, with its wide acceptance probably post-dating the founding of iNat by several years. I also believe iNat staff have made it clear that shuffling very high level taxa around would be extremely disruptive and almost certainly never going to happen, so probably not the sort of thing worth dwelling on.