Why was my drive not converted to APFS while installing macOS High Sierra public beta

I was wondering why my drive wasn’t converted to APFS after installing macOS High Sierra public beta.

I tried to convert it to APFS after the fact, by booting into Recovery Mode and opening Disk Utility, but it won’t let me. The option to convert to APFS is grayed out. Is there any way I can convert my drive to APFS?

I also tried to format my external hard drives as APFS. I was able to do that only ONCE. When I tried to do it on my second external hard drive, APFS wasn’t even offered. Does anyone know why I can’t format my second hard drive as APFS?

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.

FYI -- in beta 13 (Sept 1) -- HDD-only Macs cannot be converted to APFS. This is a bitch for testing, because as a developer I need to make a bootable 10.13 beta with APFS to test that file system.