Everyone has mentioned the license, which is the reason ZFS isn't everywhere. I just wanted to add: Don't let the license scare you away. The CDDL is actually a nice compromise between copyleft (GPL) and copyfree (BSD). The CDDL is essentially the same as GPL except the GPL applies to everything it touches (include a single GPL *.c file in your project and now your entire project is subject to the GPL). Whereas the CDDL only applies to the CDDL-licensed files themselves. So you can include ZFS in your project without making your entire project subject to the CDDL.

The issue is, the GPL wants to spread to everything it touches (including the CDDL-licensed ZFS files), which would make the ZFS files subject to both the GPL and the CDDL. But the CDDL and the GPL are incompatible, which is why Linux can't include ZFS whereas more permissively licensed projects like the BSDs can.

So it's an unfortunate situation, but ZFS' license is not "bad". It's just the GPL and the CDDL are incompatible.