My guess is because they are constantly on and recording 24hrs a day, they have to conserve space on recording devices, and I assume they do so by using black and white, low frame rate and highly compressed images. Imagine how much space it would take up to have a nice HD quality video in full color running 24hrs a day?
If some guy is robbing a bank, all they need to know is what he was wearing, what his race is, how tall, how fat, what he did, etc. You don't need super high quality video to tell all that.