Why would they if they haven't already? It's not like Canonical has a lot of money. But, yes, anyone else who Oracle might actually elect to sue won't go anywhere near ZFS.
Btrfs has also been improving. Certainly not for all use cases but it's the default for desktop Fedora now and some Synology NASs use it, among others.
On one hand ZFS has held down btrfs because "why bother" when you already have something that does much of what you want, but on the other hand it shows what can be done.
Because there's a moderate change that it goes against them.