My uneducated guess is that dividend reinvestments seem to be a win-win situation between companies and shareholders. As a result, I wonder why some dividend-yielding companies do not offer dividend reinvestment plans (DRIPs).
In the past, a DRIP made sense for small long-term investors because reinvesting dividends in an average brokerage account wasn't trivial. The brokerage would only let you buy an integer number of shares and it would charge a commission on the trade so unless you were getting a large amount in dividends, it wasn't financially reasonable to immediately reinvest those dividends. A DRIP solved those problems but imposed some costs on the company to set up and administer the plan and removed some flexibility from investors since there was more friction if they wanted to move money into a different stock.
Today, just about any brokerage lets you reap all the benefits of a DRIP just by checking the "reinvest dividends" checkbox. Most brokerages let you own a fractional number of shares and won't charge you a commission for reinvesting dividends whenever they arrive in your account. The broker takes care of all the administrative tasks that the company had to pay for in a DRIP. And the broker makes it easy for the investor to move money between stocks at will. Given that, it really doesn't make much sense for a company to start a new DRIP program.
Just to add one more argument to Justin Cave's great answer: there is no tax advantage of using dividend reinvestment plans (DRIPs) instead of using one's brokerage account own "reinvest dividends" checkbox.
The investor must still pay tax annually on his or her dividend income, whether it is received as cash or reinvested.