Linux has wider, not better hardware support. If OpenBSD supports some hardware it simply works out of the box and is stable and rock solid.
"Done is better than perfect".
While Linux has better hardware support, i usually find this to be in the more exotic direction.
Edit: support as in commercial support with SLA etc
Linux has ZFS.
Kind of.
You have the choice of the FUSE version that's legally free and clear but has obvious FUSE related performance limitations, or the kernel version which has great performance but is questionable at best from a legal standpoint because the CDDL is not compatible with the GPL.
Canonical has decided they're willing to take the risk by bundling it in Ubuntu and so far it hasn't backfired on them, but there's good reason to believe that Oracle's lawyers may have something to say about it if they ever feel that ZFS-on-Linux is threatening any of their products.
I think it's simply down to Linux being where the money is. The big players (IBM, Dell, etc) are all actively promoting Linux, and trained personel is also somewhat easy to find. So Linux is "the beast you know".
As for FreeBSD, it might be a technically better platform, but it is living in Linux' shadow.
Personally i run FreeBSD for the excellent documentation, stability, features like ZFS, but nothing i run couldn't just as easily run on Linux.