Since Btrfs was arguably available in the kernel too soon. Some people who should have avoided a new filesystem for lots of reasons ranging from missing features that were in the plan but not implemented yet or in the earliest days not yet stable... many of these early experimenters developed and fostered a vocal belief that Btrfs was "bad" (there's too much to sum up honestly, you could write an entire decent length blog post on the history of Btrfs, its reception and its criticism) and this group has taken a long time to accept that Btrfs may be ok now. Theres still people with the perception that its not as good or ready enough and should not be used.