No one knows why. Is it the infrared, or the electronic noise they make? Some think that they simply recognize them and avoid them. I have friends in Ohio who tried making camera traps where the camera is covered by bark and a trip wire opens the cover and the pic is taken. This was deep in the forest and far from hiking trails and human activity. When they went to retrieve the cameras, they were all smashed.

There is plenty of photographic evidence of Sasquatch. Discounting the hoaxes, there are some pretty good ones, but the best is still the 1967 Patterson-Gimlin film. Many claim it is a man in a suit, but modern computer enhancement says otherwise. You can see muscle movement, something a suit wouldn't show. If you're interested in it, M.K. Davis has probably the best computer analysis of the PGF.