Because Partey got to the playing distance first! It's as simple as that. However obstructive it may look, and as much of a foul it may instinctively feel, the Laws and their interpretation mean that Partey can legally occupy that space immediately in front of the ball before KDB arrives milliseconds later. If the ball was not so close to Partey's foot when he plants it down then I agree it's a penalty. However based on the TV angle from behind the keeper, you can see why VAR agreed with Oliver.
Remember, the laws are not foolproof as Rashford proved at the Swamp, and sadly can be very subjective in their interpretation!