So the story goes, this dates back to some interns who worked at NetApp and then went to Sun and perhaps influenced ZFS.
The technology in WAFL is that of a pointer-based filesystem - which itself is pretty ingenious and is only now being feature-emulated (ZFS, BRTFS, etc).
One can say what they want of Netapp's pricing, but the technology is extremely solid and simple to operate compared to managing Linux or Solaris boxes running a filesystem as a NAS; the snapshots are without I/O penalty and you can take a lot of them, the clustering is *FAR* simpler than anything happening on general-purpose OSes, the support for protocols is industry-leading (FCoE, NFSv4, SMB 2.0 - they have a codeshare w/ Microsoft and do not use a reverse-engineered Samba implementation or run any kind of Windows storage server like competitors do).
ZFS has a lot of promise, but does not have nearly the performance that WAFL does (considering RAID-DP versus ZFS RAID6) and has only some of the feature set of mirroring, snapshot vaulting, filesystem and file cloning, WORM-compliance, etc. Companies don't want to bet their business on a science project of roll-your-own NAS which doesn't have the feature set the Netapps do, and no serious competitor (eg a company with the ability to financially stand behind the product) in the enterprise space has anything like the feature set.
I work for a systems integrator and I've messed with hundreds of Netapps, Sun and Linux appliances, and competitors over the years. I use ZFS at home because I can't afford a Netapp (and wouldn't want to pay the electricity bill if I could!) but if I ran an IT department I'd put my data on a Netapp FAS over a ZFS appliance any day.