They have, but they had fallen way out of favor for a couple of reasons - write amplification, cost/complexity of segment cleaning, poor performance for some workloads, etc. They've experienced a bit of a resurgence thanks to flash and SMR, though. Personally, I've always thought log-structured filesystems were elegant and wish they'd been more actively developed during the long "winter" during which their COW/write-anywhere cousins stole all the limelight.