Before the interstate system, US-Highway 40 was one of two principal highways serving the Denver area. US-Highway 40 is Colfax. The idea for I-70 then was, not go through the heart of downtown like Colfax, but keep parallel to the north for the most direct route between Golden (and the mountains) and the airport, and to the east, for moving military or commercial goods. 

The stock yards were located along the route, the Denver Coliseum was already in existence, and use. I-70 made travel to each easier, faster, and without overloading surface streets. I-70 was built to keep traffic close to Denver, not to push the main east-west route several miles away from it, making travel times and distance longer.

Denver wouldn't look the same if I-70 had been built across Adams County. Businesses would've located near it, to take advantage of it, and the potential growth with it.