Also do you know why FS kept the low level non radar airspace and ATC the radar airspace.
There was not radar or non radar airspace. Excluding military there was only Controlled Airspace (CTA), and everything else was Uncontrolled Airspace (where you were literally OCTA - Outside CTA). ATC provided ATS in controlled airspace whether radar or not, FS was available in everything else under, beside,
and above the CTA. Depending on where they were, aircraft could be at any level and be OCTA.
ATC had their airspace before they had radar. Radar was just a tool to help them do their job better. FS didn't need it for the level of service they provided, so didn't get access to it, and as has been pointed out, there was no coverage in the vast majority of the airspace they were responsible for.