High humidity has a couple of affects that are bad for imaging.  The first, and most important, is that high humidity tends to also place you close to dew point.  Any water vapor, misting, or condensation brought on by humidity or dew can cause a massive number of problems with the telescopes and CCD as these are fully motorized and electric machines.  The dew and condensation can ruin computer parts that run the telescope, affect the mounts, and layer the mirror with frost due to the CCD cooling (most are run at -15ºC or lower)

The other side is that high humidity means there is a thicker atmosphere to gather light through.  When imaging during high humidity the light not only has to pass through our atmosphere, but also through the higher density of water particles in the air, which just like a flashlight on a water droplet, will scatter the light on it's way to reaching the telescope.  This lowers the seeing drastically and will lead to very noisy images, guiding problems, and the occasional pointing error.  

In short, it makes the system very unreliable and about 95% of the images taken will need to be refunded, making it not worth the risk of exposing the systems to possible water damage. High humidity means poor seeing.