[slodave]

After a little research. We were always told it was because of the tourists.

After repeal of Federal speed mandates in 1996, Montana was the only state to revert to rural daytime speed limit, beyond the Basic Rule. After the Montana Supreme Court decided that the Basic Rule was too vague and therefore violated the due process requirement of the Montana Constitution,[35] Montana's legislature imposed a 75 mph (121 km/h) limit on rural freeways in 1999, although the same wording in the basic rule remains.