US 287/89 has less traffic than Hwy 83. Also, Hwy 83 has limited shoulders in many places.
US 287/89 has wide-open mountain vistas - Hwy 83 is largely forested with occasional views.
Obviously if one is coming from Missoula it is further using US 287/89.

Montana DOT traffic:
http://www.mdt.mt.gov/travinfo/docs/bike_map.pdf

The 2430 figure for Hwy 83 is a the low point - around Seeley Lake and Swan Lakw it may push 5000 ADT in summer.
US 287 has an ADT of 540 - sublime. US 89 has 1400 ADT tops - plus much of the route has good shoulders.

It's an issue of quality over quantity.