Why doesn't the time zone demarcation follow the Colorado/Kansas border, instead of slicing Kansas in two?

When the U.S. Department of Transportation got into the business of deciding where to draw the lines for time zones back when railroads started crisscrossing the country, that line between Central and Mountain time shot almost straight through the middle of Kansas, about 125 miles east of where it runs today.

Over the last century that line has edged westward and even back and forth and north and south a couple of times. As late as 1990, Kearny County, Kan., was switched back to Central Time from Mountain Time by the Department of Transportation after a series of community meetings.

But what's almost more amazing than anything is that the folks (approx 11,000 residents) in these four counties like their Mountain Time. Twice in recent history - 1990 and 2002 - residents have voted 3-to-1 in favor of keeping their Mountain Time--to keep in line with Denver, CO time.

http://www.rockymountainnews.com/news/2008/nov/01/clock-change-a-daily-challenge-in-part-of-kansas/