Lets say Nokia's chances of success are 20%, but you have the ability to improve them to 30%. Suppose your pay package is 100% incentive based, you only get paid if Nokia survives. Why would you take this job? You have a 70% chance of getting nothing. It's much easier to go back to MS, and get a guaranteed paycheck.
In a few years, it will be easy to blame Stephen Elop and claim it is unjust for him to be paid even while Nokia tanks. But it isn't his fault that Nokia is in trouble (he's been CEO for 13 days) - he might be their best hope for survival. If he wasn't getting paid cash up front, he would probably just turn down the job and Nokia would be in even worse trouble.
(Not trying to endorse Elop, I know very little about him or Nokia. Just putting a face to the situation rather than speaking in the abstract about CEO A and company X.)