Simply put: because there isn't enough "somethingness" to fill it up (with structures). Only a limited amount was created and since the universe has been constantly expanding, there has been more and more room for the particles to be in so it's not a surprise to find things to be far apart.
Why the universe is not homogeneously 'filled' with these particles is an important related question$^1$. Small fluctuations in the density of the early universe sparked the birth of huge structures due to gravitational reinforcement. A slightly denser part of the early universe has a slightly higher gravitational pull, so more particles are attracted to that region. More particles in that region means an even bigger abberration from the mean density of the early universe, so this thing is self-reinforcing. And so structures are born. Stars come into existence.
Eventually these stars die and many of the elements they produced during their lifetime are thrown into space. These heavier elements (after a few star generations) then give rise to the birth of planets and other celestial objects, but that's getting off-topic (and it's a bit more complicated than that). The point is that there is a limited amount of matter present. So since the universe has been ever-expanding, it's to be expected that structures are 'few' and far between.
$^1$ Another very interesting one is why we see so little antimatter in the universe, but I think there are already a few questions about this on here. (for example, this one)