The complaints about noise from residents along the southwest side of the loop was a red herring. The real reason why I-40 was moved back onto its previous route through Greensboro was so that NCDOT wouldn't lose Interstate Maintenance funds.
I know this is the explanation Jim Dunlop pushed in MTR, but it doesn't convince. NCDOT had the option of applying for a concealed Interstate designation for Business 40 which would have maintained eligibility for IM funding--this is what California does with Business 80 in Sacramento (hidden Route 51, hidden I-305). Moreover, NCDOT should have, and probably would have, considered the implications of IM funding before relocating the I-40 designation to the bypass in the first place.
It is, I think, relevant that the bypass route was longer than the original I-40 routing, and was not attracting I-40 traffic aside from trucks not wanting to go through Death Valley. The shift in designation looks like a belated acceptance of reality.