Extended attributes aren't limited just to Solaris. A variety of *nix file systems have generally supported forks/multiple data streams for some time. FreeBSD has support for arbitrary-sized extended attributes in UFS/ZFS, for example, and implements its access control with them. Linux probably has the most limited form, where the xattr are capped at 4 or 64 kilobytes depending on the underlying FS.
I think the big issue for uptake is that it just doesn't fit into the UNIX world. Nothing else is aware of xattrs/forks. The basic file utilities will blindly strip them so you can't just cp or tar such a file, for example. If cp transparently breaks all your files, that's pretty much a show-stopper as anyone who remembers the classic Mac to OS X transition will know.
The reason I mentioned Solaris is because Solaris behaviour is closest to NTFS, whereas linux API is based upon Irix, which is closer in behaviour to OS/2 (OS/2 subsystem was another use case for ADS), as well as implementation on a bunch of other systems where extended attributes were treated as just that, attributes.
Comparatively, NTFS is very... flexible navigational database and a bunch of things are just "this is the default parameter set, not exhaustive".
I wasn't aware that FreeBSD copied Solaris in this (ZFS extended attributes by default create a sort-of directory object allocated linked from the file, linux supports inlining simplified xattrs for speed)
There is a difference between alternate data streams and extended attributes. Basically, the former is data, the latter is metadata. NTFS and ZFS implement alternate data streams, but in eg FreeBSD those are accessed like extended attributes.
NTFS doesn't really have a difference internally - they are all attributes of the FILE object (everything in NTFS is a FILE or its attribute, including filesystem metadata). Arguably ZFS ZPL has "blessed" data element (I dare you to implement ZFS while replacing DMU/SPA with Ceph or S3 XD - yes it's doable)
I think the big issue for uptake is that it just doesn't fit into the UNIX world. Nothing else is aware of xattrs/forks. The basic file utilities will blindly strip them so you can't just cp or tar such a file, for example. If cp transparently breaks all your files, that's pretty much a show-stopper as anyone who remembers the classic Mac to OS X transition will know.