Why? IIRC running ZFS on NVMe SSDs limit their performance seriously. With sufficient queue depth modern SSDs can easily get up to 1mln+ IOPS and on ZFS I can barely get 100k :(
Most probably because ZFS still has the most extensive data correctness guarantess of all filesystems on Linux. Yes, bcachefs will have checksums too but it it is still in beta.
copy on write, instant snapshots, data checksumming, easy rollbacks, auto trim, compression
proxmox takes advantage of zfs' features if available, like when you select block storage on zfs it makes each vm disk its own dataset which you can tune as you see fit for each one. cloning a vm is instant if you make a cow clone. suspending the vm to do snapshot based backups is also nearly instant, etc
Disable atimes and turn off the arc cache and metadata cache (don't really need either with fast storage compared to spinning rust) and use lz4 compression and you minimize write amplification or needless wear on the drive. zfs is perfectly fine on ssd for my use case