Mostly target audience, I'd imagine. Anyone still using NFS or iSCSI probably has specific performance or integrity requirements that those services aren't gear

Mostly target audience, I'd imagine. Anyone still using NFS or iSCSI probably has specific performance or integrity requirements that those services aren't geared for, plus multiple concurrent access that makes locking and cache coherence significant questions. In terms of low-urgency storage though, the Dropbox/Google Drive/Syncthing model of directory replication largely has supplanted SFTP, though. There is probably some middle group of S3/B2 type storage backing some FUSE-style integrated filesystem, but I'd suspect the same problem that makes WebDAV unpopular hampers that, namely that it almost invariably sucks horribly on Windows when it works at all.