I'd guess one issue is the number of items. Radial probably only works for 6 or less. With a scrollable list, any number would work (And many popups have a variable number of options).
Agree, it's very handy and natural .. also existed in many forms.
The lack of seeing this may be an patent issue where a patent for a radial menu needs to be invalidated. Pie menus existed back in the 60's, before the patents, lots of examples in this thread.
And for what it's wroth, I'm glad this thing exists, because people should be able to see it and say 'oh yeah, this is like the buttons on my remote control"
Radial menu for some reason triggers my OCD. It feels out of place, where all the menus are square. If this was an option I would have that turned off. It is also obscuring whatever there is around the cursor. Reminds me of autocompletes where they cover the line below that has information I need to see whilst typing. So annoying.
In the happy wild times of custom ROMs frenzy on Android (around versions 2.6-4), there was plenty of such attempts. Many devs included them directly in ROM by default.
Unfortunately, all I saw were rather dirty hacks that only kinda worked.
This assumes the phone knows what angle your finger is coming in from. Even if you assume it is coming in from the bottom half that introduces problems when the hit point is near the top of the screen.