Why is crustacean a group but fish is not?

@yongestation is correct. At this point, the iNaturalist database is so large it is virtually impossible to move around any high level taxa, sometimes even down to family. @loarie has said that they are looking into addressing this, but it will “not be addressed for a while.” iNaturalist is not intended to be a taxonomic authority, although unfortunately it is gradually becoming one simply by virtue of being the only platform that is kept up to date (on lower level taxonomy at least). Hopefully sometime in the glorious future we will be able to manage the higher level taxonomy again.