Unlike private companies, publicly traded companies stand vulnerable to takeover by another company if they allow their share price to decline substantially. This exposure is a result of the nature of ownership in the company. Private companies are usually managed by the owners themselves, and the shares are closely held. If private owners don't want to sell, the company cannot be taken over.
Publicly-traded companies, on the other hand, have shares distributed over a large base of owners who can easily sell at any time. To accumulate shares for the purpose of takeover, potential bidders are better able to make offers to shareholders when they are trading at lower prices. For this reason, companies would want their stock price to remain relatively stable, so that they remain strong and deter interested corporations from taking them.