The tapered thread makes a much better grounding connection then does a straight thread. As far as I know factory conduit threads have always be NPT. Not sure when the rule for field cut tapered threads went into the code, but I would bet that it was put in to require the field cut threads to match the factory cut ones.
I do know that about 20 years ago that the Canadian code required tapered thread in the conduit couplings as well as on the conduit threads. When the conduit manufactures got the code changed so that they could supply the cheaper straight thread couplings, the electricians wanted to know why their conduit "leaked" in wet locations more than it did before.
