I ran into that issue with an install last summer.
My install reused an existing partition, which was originally using “ext4”. I told it to use “btrfs”, which it did but without snapshot support. Apparently there was some box I should have checked to allow snapshot support, but I missed it.
I reinstalled, and this time it did default to snapper support.
Apparently: “ext4” → “btrfs” the default install behavior is to not enable snapshots. But with “btrfs” → “btrfs” the default is to enable, even if the original did not use snapshots.