Eastern Montana is flat grass and range land with few recreational opportunities. On US2 that extends all the way to Glacier. I94 follows the Yellowstone River to I90, and then passes through settled valleys and passes all the way to Idaho. So you hit the mountains sooner, as well as the recreational options that are a spin off of the mountains (National Forest), towns and Yellowstone. Since you already did Yellowstone and Glacier, you presumably have an idea of what's available along the mountain spine that connects the two.