Why do companies delist from stock exchanges ?
As explained here.
A company delists for 2 reasons - forced or voluntarily.
Forced delist - This is when the exchanges decide to delist the company, usually for non-compliance. If you are holding shares of such companies, then you will have to write it off as bad investment.
Voluntarily delist - This is when the company itself decides to delist the shares from the exchange. This could be an outcome of a corporate restructuring exercise. Under such a situation, the company has to offer a premium over and above the current market price of the shares. Usually, the premium shoots up to 30-40% or more.