Because the universe is expanding. Space itself expands the further away you are from a reference point. That means, everything is moving away from us (on a grand scale), and the greater the distance to the reference point, the bigger is the rate things in the universe are moving away from us. I think the rate is about 70 km/(Mpcs) - wich is an incredibly small number, scaled down to a meter it is ~10^-23 m / (ms). And as space expands over time, the density of matter in space get lower and lower.
Space isn't an empty vacuum, but that's tangential. The expansion in this context is metric explanation, which just means things are getting further and further apart between two points due to space "expanding" between non-gravitationally bound objects. The explanation for "why" is sort of "we don't know". AFAIK, the leading theory is a combination of momentum / inertia from the big bang and acceleration from dark energy. Which we also don't know a lot about.
