This is because Martin Matuška of FreeBSD has been focused on upstreaming changes made in FreeBSD's ZFS port into Illumos. At present, the ZFSOnLinux project has had no one dedicated to that task and code changes mostly flow from Illumos to Linux. This is starting to change. A small change went upstream to Illumos earlier this year and more should follow in the future.
That being said, there are commonalities between Illumos and FreeBSD that make it easier for the FreeBSD ZFS developers to collaborate with their Illumos counterparts:
1. FreeBSD and Illumos have large kernel stacks (4 pages and 6 pages respectively) while Linux's kernel stacks are limited to 2 pages.
2. In-kernel virtual memory is well supported in FreeBSD and Illumos while Linux's in-kernel virtual memory is crippled for philosophical reasons.
3. FreeBSD and Illumos have both the kernel and userland in the same tree. FreeBSD even maintained Illumos' directory structure in its import of the code while ZoL's project lead decided to refactor it to be more consistent with Linux.
Difficulties caused by these differences should go away changes made in ZoL to improve code portability are sent back to Illumos.
That being said, there are commonalities between Illumos and FreeBSD that make it easier for the FreeBSD ZFS developers to collaborate with their Illumos counterparts:
1. FreeBSD and Illumos have large kernel stacks (4 pages and 6 pages respectively) while Linux's kernel stacks are limited to 2 pages.
2. In-kernel virtual memory is well supported in FreeBSD and Illumos while Linux's in-kernel virtual memory is crippled for philosophical reasons.
3. FreeBSD and Illumos have both the kernel and userland in the same tree. FreeBSD even maintained Illumos' directory structure in its import of the code while ZoL's project lead decided to refactor it to be more consistent with Linux.
Difficulties caused by these differences should go away changes made in ZoL to improve code portability are sent back to Illumos.