I read something in the past that offered some details of what he was supposed to get and it was in line with Vettel's contract. What is being reported as his salary this year is, I think, not entirely accurate.
Then, Kimi does have a little money already and may have been just as interested in "less
PR bull****" so the final dollar figure might not have been the deciding factor. It could also be his driving style and how that might have not meshed with a car he knew was going to be tailored to Vettel.
JR