With United, he could fail to make any impact, keep them in Spurs' shadow for two or three more seasons, and then get sacked at the end of it.
Why take the risk when he's already well on the way to achieving what he wants at Spurs?
We are leagues ahead of United, and Poch will know that with them now being a rudderless ship that he can push on even harder and further.
At Spurs, Poch can be a legend. At United, the most that he can realistically hope for is the odd foray into a domestic cup final.
I think the man has greater ambitions than that, quite frankly.