The same piece of confectionery (chocolate, nougat, caramel) is known
in the United States as a Milky Way bar, and in the UK (as well as
Canada and many other countries) as a Mars Bar. Given the company that
owns these brands is the Mars corporation, and that it was started by
Frank Mars, what was the original reason that the bar didn't get named
the 'Mars Bar' in the U.S.?

Hershy ?

Pedigree Pet Foods ?

Actually in the UK a 'Milky Way Bar' used to be very different from a Mars Bar

Somebody will pop up and explain the father/son empire.

Perhaps this can help:
http://en.wikipedia.org/wiki/Mars_bar

Are you sure that a Milky Way and a Mars bar are the same? I
mean...have you actually tasted both? I looked at a picture of a Milky
Way bar on the American website and they have one cut open and it
looks NOTHING like what a MARS bar looks like! To me a Mars bar has
always seemed like a Snickers without the nuts or like a Three
Musketeers with a caramel layer..try looking at some pictures to get
an idea....

I'm not sure if the forumlation changed over time or not...the Mars
website does not indicate if a unique formulation was derived for
Europe/Canada vs. US.

Some decent info has already been put together on this topic at Wikipedia...
Looks like Mr. Mars used Milky Way first and when it became popular
decided it to launch it outside of US with is own name on it (and
launch a bar in the US with his name on it too but with a different
formulation)....

http://en.wikipedia.org/wiki/Mars_bar

The sales pitch for the product was that it tasted like chocolate
malted milk, and "Milky Way" says that better than "Mars Bar."

http://www.milkywaybar.com/facts/funfacts.html