Wondering if anyone knows when the government decided it would be the lower portion of the receiver that would be registered and not the upper, and what the rationale was?

If I recall, it's the upper receiver on the FAL and HK that is registered.

In for answers.  I've always wondered why the lower is the serialized part on the AR, but the upper is the serialized part on the 10/22.

ETA See https://www.atf.gov/firearms/firearms-guides-importation-verification-firearms-ammunition-firearms-verification-overview.  The serial number is supposed to be on the "frame or receiver".  27 CFR 478.11 - Meaning of terms, defines the frame or receiver as "That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel."  The serialized part on both the AR and the 10/22 "provide housing" for the hammer and the firing mechanism.  Based on what I've read since I initially replied, the definition is broad enough to give manufacturers and importers some flexibility as to where they put the serial number, which explains the inconsistencies.

None of this mattered until 1968, so the location for the AR-15 was determined by the manufacturer, not the ATF, and then grandfathered in. Probably the military had a lot of say in it for guns built for or marketed to them.

Since then, it's been the portion of the firearm that held the barrel or trunion, and often where the ammunition feed (magazine or belt feed) was located.