The only reason why Button is there is because Whitmarsh was able to get him a lot cheaper than Kimi.
Whitmarsh was all goo goo gaa gaa over Kimi in the Abu Dhabi 2009 build up...He gave Kimi and the Robertsons a 200 page contract at that meeting to look over...and then Button offered himself cheaper without an interest in doing Rallys on the side.
Button isn't very complicated. That's a huge plus.
Hamilton may be faster but he's more "complicated" for a lack of a better word...and he isn't as "patient" in the races, for lack of another word.
"patient" could be a word that applies more to Button than it might to Hamilton. "aggressive" might be a word that applies more to Hamilton than it might to Button...so, perhaps, Whitmarsh likes 'patience' more than he likes 'aggression'? I don't know....I'm merely asking. He's from engineering - as opposed to business development or marketing - so may be he likes a patient/"methodical" driver. Business folk are more "agressive"...kinda like Hamilton.
So this could be a factor. A 'style' thing. I don't know...
...What I do know is that Lewis is Dennis' doing...he's Dennis' project...Button, on the other hand, was signed under Whitmarsh's watch...under the new regime.
I imagine Whitmarsh would be the same why if it were Kimi there as opposed to Button because Kimi/Jenson would have been/was the 1st driver he ever signed as McLaren boss.
I think that's significant.