The fact is that there are relatively few NAS [network attached storage] vendors compared to SAN [storage area network] product vendors. There are dozens of vendors selling block-based storage but just a handful that sell NAS -- and there is only one NAS vendor out there that currently does over $1 billion in revenue. In short, the storage ecosystem today is much more SAN-oriented, so there isn't as much traction for NAS products yet....
More users are adopting SANs because there are many more vendors selling it. I think that has a lot to do with it.
SANs do have the advantage of interoperability; SANs work in any environment and application. For example, Microsoft doesn't support Exchange on proprietary file systems, which is what NAS uses. So people aren't going to implement email, a fundamental enterprise application, on a NAS. As another example, you can run databases on NAS, but traditionally they are run on a SAN. NAS is 'smarter' with its file-level awareness, but that awareness also introduces some latency that limits NAS performance relative to a SAN. All this forces you to think a bit about where you should use NAS vs. SAN.
Check out the entire NAS FAQ guide.