Chris,
There is no straight answer to your question....
We can only guess at the reasons why the NFPA people did not include natural gas fueled drivers in thier NFPA-20 standard.
I am guessing here, but I suppose that it may have something to do with the reliability of the fuel supply. Somehow a tank full of distilate is more reliable than a gas main.
When NFPA codes and rules apply to a project, they must be taken, "in toto"... the picking and selecting of rules is not permitted.
-My opinion only
-MJC