As alluded to before, the lower center of gravity and lower weight (by nearly 400 lbs it seems) producing better handling.
Much smaller frontal area, for improved highway efficiency/range. Yes, the EPA combined range is nearly identical, but this is taking into account the added efficiency of the heat pump in cold weather testing, which is less important on long highway trips, where range actually matters, and completely unimportant when cabin heat isn't needed.
Potentially cheaper tires - smaller sizes on smaller wheels tend to be cheaper - and reduced weight reducing cost of operation.
Physically smaller, so easier to park. (Y is significantly bigger in every dimension.)
(Personally, I want something with a hatch, but don't want anything anywhere near as big as a Model S or Y.)