The reason is the signal has issues. The macrovision copy protection and the VCR may degrade the signal.
If you were trying to tape a DVD movie, then that is specifically degraded so VCRs can't or either do a very poor job.
Have tried to hook up 4 DVDs at my house by running the video,audio through a VCR which is hooked to my TV. Three of these were the same brand and one was another brand. Each time the picture on the screen became worse and worse with white streaks coming across the screen and the picture alternating between light and darker colors. Am using a very strong surge protecter so I do not think there was any kind of electrical surge to damage the DVD player but we are stumped after four players as to what the problem is. The same problem has occured each time I have tried to play a DVD and it gets worse the longer the DVD plays. Need feedback.