If the body is galvanized but not the frame I could see that being a reason to isolate the two.
Theoretically if they were connected, the galvanization on the body would protect both the body and the frame at first, assuming enough paint has chipped to expose the galvanization.
However, if the paint only chips in a small area on the body, the larger area of the frame could result in the galvanization at the chip being lost very quickly, resulting in significant corrosion at the paint chip.
The total overall amount of corrosion of steel would be the same regardless of whether or not they are isolated, but by isolating them you sacrifice frame metal to save body metal. Maybe they assumed a small amount of frame corrosion is less damaging than an equivalent (by weight) amount of body corrosion. Which would make sense given that the frame is much thicker.
If neither are galvanized I’m not sure there would be a reason to isolate the two.