What a fascinating question!
Turns out it's less flammable.
Ground Supply Fluid—Because the flash point of RP-1 fuel, which
supplies the system in flight, is 110 to 139° F, it is classified as a
Class Ill flammable liquid, not suitable for ground operations. A
study was made to find substitute fluids with properties similar to
RP-1 that could be used in the laboratory and in ground operations.
Such a fluid was found in RJ-1, a ramjet fuel. It has properties very
similar to RP-1, but a flashpoint ranging from 190 to 208° F. If
purchased with a specified flashpoint exceeding 200° F, it is not
classed as a flammable liquid by the National Code and would not have
the same stringent handling requirements of RP-1 fuels. Therefore, the
RJ-1 fuel can be used in the laboratories or in GSE with the same
precautions as taken with MIL-H-5606
GSE = Ground Support Equipment
Source - The December 1963 issue of Hydraulics and Pneumatics magazine