Why is there not a significant rain shadow effect east of the Rocky Mountains?

There is a significant rain shadow effect east of the Rockies. One prime example is Denver. Over the course of a year it would typically receive ~20 inches of rain, but many locations to the west of the continental divide receive as much as 40 inches of rain a year. (It's important to note that the rain shadow effect only impacts storms and winds traveling against the mountains. If a storm travels south to north, the effect is severely diminished and can result in a situation where the lee side of the mountains receives more rainfall than the wind side of the mountains.