Because they still haven't fixed the write hole issues in certain RAID configurations,which have been known for a decade or more.

A project not finishing some feature is not the same as being abandoned. It seems lots of people are happy to use btrfs in production without that raid mode. In other words, for all the complaining about raid5 that happens every time btrfs is mentioned, you'd think there would be at least one person who cares enough to implement it. Yet people use it in production and keep improving the other parts of that project instead.

It's fine for single a single disk or something that presents as a single disk, like a SAN. RAID1 seems fine also. I really wanted to love it, but after it ate my data a couple of times, I gave up trying to use it for mass storage. At that time they didn't have warnings in the documentation and had stated that RAID5/6 were basically complete.

I found using mirrored vdevs in ZFS much easier to manage and much more stable.

> I found using mirrored vdevs in ZFS much easier to manage and much more stable.

That's not exactly a fair comparison. If you restricted your usage of btrfs to a similarly narrow range of features, you would probably have had a much better experience.

RAID1 in Btrfs is not entirely fine. It won't nuke your data and there is no write-hole issue, but if a disk fails you'll have to go into a read-only mode during the rebuild and deal with various hurdles in getting it rebuilt.

> but if a disk fails you'll have to go into a read-only mode during the rebuild and deal with various hurdles in getting it rebuilt.

I can't say for sure that this never happens, but that's certainly not been the failure mode for any of the drive failures my btrfs RAID1 has experienced. I don't think I've ever needed to reboot or even remount my filesystem, just replace the failed drive (physically, then in software). But I always have more than two drives in the filesystem, so a single drive failure only puts a fraction of my data at risk, not everything.

> if a disk fails you'll have to go into a read-only mode during the rebuild

That's not correct, you can rebuild online. The readonly mode is only relevant when you reboot during the failure and don't have the right options set on the volume.

But you totally can replace a live drive without affecting the availability.

> I can't remember the last time a kernel was released without something at least a bit exciting about btrfs

They are fixing the fixable issues, but the on-disk format still makes some gotchas inevitable. It sounds like there's never going to be a great solution to live rebuilding of redundancy.