Some of us have a theory that there is a finite resource of mental energy that can be entirely spent on strong intention during the day, that leaves nothing left for night-time lucidity. That explains the "relaxation" effect which many people have noticed -- a lot of people get lucid right at the end of a LD competition, for example. You still had strong awareness buildup from all the earlier practice, which does not simply vanish, it's like building up muscles, it takes a while of inactivity before they start to atrophy.

In some ways, its like "trying to sleep" -- which usually results in insomnia. As with just about everything in LDing, there is a fine balance to be found between trying too hard and not trying hard enough.