Delisting
The removal of a listed security from the exchange on which it trades. Stock is removed from an exchange because the company for which the stock is issued, whether voluntarily or involuntarily, is not in compliance with the listing requirements of the exchange.

Listing Requirements
In other words, if a company messes up, the exchange will kick the company out of its exclusive club. A stock that has experienced a steep price decline and is trading below $1 is very risky because a relatively small price movement could result in a huge percentage swing (just think - with a $1 stock, a difference of $0.10 means a change of 10%). In low volume penny stocks, the fraudsters flourish and stocks are much more easily manipulated; major exchanges don't want to be associated with this type of behavior, so they delist the companies that are liable to be affected by such manipulation.