Why was my drive not converted to APFS while installing macOS High Sierra public beta
According to various threads I’ve read, there are a number of factors that may prevent you from converting to APFS:


Third-party SSDs are not supported.
Rotational hard drives and fusion drives were supported in Developer Preview 1, but support was removed in Developer Preview 2 (Public Beta 1) due to catastrophic failures.
FileVault may need to be disabled before the conversion is allowed.
External drives may need to be unmounted.