Authorities blame the pot smokers. While stealing a sign is still frowned upon the easiest solution to put a stop to what was happening was for the Colorado officials to think creatively. The term "420" is very common nowadays among pot smokers and is a code for the drug. Its origins are not clear but it is believed that this code was first used among a group of California teens.
The solution Colorado officials took is rather ingenious. They decided to slightly move the sign up the road and to change its text to the new one. It appears that the Colorado "MILE 420" is the only sign that was replaced so far because most highways do not extend out to the 420 mile marker point.
Officials did not take credit for the idea though. They made a reference to a "MILE 69" marker which was replaced after numerous thefts with a "MILE 68.5" sign. In this case as well authorities believed the increased thefts were due to the fact that some people might have took as reference the sexual practice when they saw the sign.