Empire spoke to Mendes recently and this is what he had to say:

It has been a very difficult decision not to accept Michael and Barbara’s very generous offer to direct the next Bond movie. Directing Skyfall was one of the best experiences of my professional life, but I have theater and other commitments, including productions of Charlie And The Chocolate Factory and King Lear, that need my complete focus over the next year and beyond.

The people Sam Mendes mentions are Michael G. Wilson and Barbara Broccoli, who were producers on Skyfall and other Bond films. They contacted Empire shortly with this response:

We thoroughly enjoyed working with Sam, he directed our most successful Bond movie ever, Skyfall. We would have loved to have made the next film with him but completely respect his decision to focus on other projects and hope to have the opportunity to collaborate with him again.

Source: JoBlo