marketing. iirc his manager is the same guy behind beckham's move to the galaxy. mercedes is a much, much bigger brand name than mclaren (outside of f1). mercedes might not be able to make his basic wage massive, but there are ridiculous marketing opportunities available outside f1 that trump mclaren. imagine being the face of a brand like mercedes.
Vettel and McLaren looking quick over practice 1,2&3. Alonso looks like he has something up his sleeve, in his hat or hidden away in his cape! Hülkenburg looked quick in P3 also. Think qualifying and the race itself will be pretty close.