It is not universally preferable. Some thinkers, often following Nietzsche, encourage having as much influence as you are able to tolerate.
(The more evolved in this crowd soften that to 'as you and the system are able to tolerate honorably'. Neitzche wrote boldly about the 'value of crime' and the 'power of the lie', but that is not how he lived. In reality, he considered crime 'in poor taste' and lying a waste of everyone's time. We all realize that social norms have value.)
He/they would blame the preference for passivity on our culture's choice of 'slave morality', influenced by Christianity and similar threads in other religions, and by the complexity of our modern cultures. We exonerate innocent victims of circumstance, instead of taking the older path of demanding choice as a matter of honor, because 1) we are culturally invested in the 'magic' of the paradox of strength through weakness and 2) we are also used to being in situations where we lack a significant part of the necessary information.
So we can see ourselves as innocent bystanders, forced unwillingly into the situation we are not ready for, and as wise agents avoiding acting on a potential misunderstanding. That gives us a defensible ethical position. But in the end, if we go down that path too consistently, we are drawn into the position of lionizing victimhood, which is ultimately bad for everyone, since victimhood is not a productive value, and actively produces weaknesses and attendant social costs that need not exist.
That set of folks would kill the fat man if this were a split-second decision. But from an even more powerful position, they would try to convince the fat man to do himself in.
Respecting autonomy is always a more powerful position, because, you have deployed not only 'power over' the situation, but 'power with' those with more direct power (to borrow terms from Starhawk). In this case, if you get your way, you have made the decision about the outcome, but have also influenced the fat man himself directly toward heroism.
If you don't get your way, you have taken the position with the higher potential return-on-investment, though also with the greater risk. The more audacious proposal is better because taking responsibility and facing risk has a positive value, which partially offsets the potential greater likelihood of success in a more calculated approach.