Planet F1 (from an article in Bild Magazine) are suggesting Mercedes change of priority is simply down to money as the have "smallest" budget of the major teams in F1 at €180 million.
http://www.planetf1.com/news/3213/6309880/-Finances-dictating-Merc-s-switch-of-focus-
http://www.planetf1.com/news/3213/6309880/-Finances-dictating-Merc-s-switch-of-focus-