Banner blindness is caused by repeated exposure to multiple advertisements without any new type or format of advertisement coming in. This creates a sense of what's called 'advertising fatigue,' and people will eventually ignore banners.
An average person sees around 10,000 banner ads per day. After seeing the same ad multiple times, people start to ignore it; people are so used to seeing them that they have become 'blind' to them. The human brain is actually designed this way so that we can focus on other things.
ad fatigue
Banner blindness isn't just a problem with web banners but all types of advertising, including television commercials and print ads.
In the same vein, people develop what is called 'ad fatigue' when there is too much advertising. The human brain can only process so much information at a time, and when it's constantly barraged with ads, people start to tune them out.