Apparently the humidor was installed in 2002, and it looks like MLB allows it because it brings baseballs within specifications. Here's an excerpt I found:
It's interesting that park factors have been decreasing in recent years. Most people attribute this to the humidor that was installed at Coors Field in 2002 to keep the balls at 70 degrees and 50% humidity. There was an great article by Mike Klis of the Denver Post that talks about the humidor. I found the following interesting.
"The Rockies believe the pitching problems at Coors Field could be at least partially corrected if the weight specifications MLB has for its baseballs weren't contradictory to its humidor/ball-shipping specifications. By rule, a baseball can weigh from 5.0 to 5.25 ounces and measure from 9.0 to 9.25 inches in circumference.
This is not an insignificant variance. MLB's own study at the University of Massachusetts-Lowell in 2000 found that a well-struck, 5.25-ounce, 9.25- inch ball could travel up to 49.1 feet less than a 5.0-ounce, 9.0- inch ball.
The current humidor settings, however, cannot sufficiently offset altitude's effects to pump the baseballs up to 5.25 ounces. According to Kevin Kahn, the Rockies' chief of ballpark operations, and humidor engineering head Tony Cowell, the balls put in play at Coors Field weigh about 5.12 ounces, on average. "
According to Cowell, prior to the humidor balls were being put into play with weights as low as 4.6 ounces and 8.5 inches.