Hillbilly nailed the primary reason that pronghorn tags are so difficult to draw in Colorado. There are relatively few units with public access. As jims mentioned, the recent bad winter did slightly affect some areas in the state, but for the most part, those areas were over objectives before that winter. In fact, statewide, the pronghorn population is currently over 8,500 animals over objective.
I believe there are two other reasons that are making it even more difficult to draw these days. First, pronghorn hunting has become much more popular in this age of easily gathered information. In the 10 years between 1999 and 2008, pronghorn applications increased about 9% in Colorado.
The second situation making drawing a tag so difficult is the large percentage of tags that are issued in the public land units which are Private Land Only tags. For example, for the regular rifle season in unit 3/301, arguably one of the more popular public land units in the state (it took 12 points for a resident to draw in 2008), the DOW issued 55 unit-wide buck tags in 2008 (8 of which went to landowners in the form of vouchers). But the DOW issued 850 PLO buck tags for those units! So your options are four: you own hunting land in those units, you know someone with land who will let you hunt, you pay for hunting access, or you wait 12 years to draw a tag.
In most of the eastern units of the state, a person could hunt every year or every other year if they can find private property to hunt.