I guess the idea is that jumping for /distance/ or height is athletics, the speed/power of high STR help with that. Acrobatics, you might be able to jump high/far with the help of a springboard or a strong partner in a performance, but it's primarily about agility rather than power. Of course, players can try to get around such things. You may not be able to long-jump with Acrobatics, but you might be able to swing or do something improbable like wall-running in a video game or whatever you can sell to the DM.
Standard answer is just "because: magic." Magical grease is just slipperier than slippery. Zero friction? It has negative friction. It conjures invisible grease grimlins who push your feet out from under you. &c.
(A more mechanical answer might be "the specific rules of the spell supersede the general rules of the skill.")
But, if you're the DM, call for the roll that makes sense to you. Letting a rogue roll DEX + prof because he's proficient in Acrobatics instead of DEX + prof because he's proficient in DEX saves isn't exactly going to break the game - even if his Expertise applies, or it's another character who happens to be proficient in acrobatics but not DEX saves, it wouldn't be the end of the world.
You can also consider the way actions are declared. If the character tries to cross the grease spell normally, he makes the save. If the character's been established as being from a region where there's ice skating, and has acrobatics, OTOH, and describes attempting to slide across the grease as if skating on ice....