Beanstalk Control wants to resolve Trinisphere, Karn, the Great Creator, and The One Ring to lock us out and bury us in card advantage, so Force of Negation lets us stop these noncreature spells for free on their turn, ideally before Trinisphere is in play and makes the alternative cost expensive. Prismatic Ending stays in the sideboard: with only white and blue mana its converge value tops out at 2, so it can't exile Trinisphere, Karn, or The One Ring, and it can't target their lands at all. Stifle can't touch their lands' mana abilities, so it has fewer profitable targets than usual and we trim two copies to make room for the extra Force of Negation.