The real answer is, of course, why not? As physicist Sean Carroll notes so well in this blog post, "First, we would only even consider this an interesting question if there were some reasonable argument in favor of nothingness over existence." There is no reason we have, or really can have, to favor one assumption over the other. Why shouldn't there be something? Why is it surprising that there are things? Why is it more surprising or unnatural that there is something than it would be surprising or unnatural if there were no things? What does such a thing even mean? Certainly we can't, from our perspective as things within a universe where we are all somethings, propose an argument (something, which itself would probably reference lots of other somethings) which should lead us to believe that a universe with nothing is somehow 'preferable'. Again, quoting Sean Carroll, "We only have experience with one universe; there is no ensemble from which it is chosen, on which we could define a measure to quantify degrees of probability." Indeed.
Really, we can't even conjure a good explanation of what that nothing would be. In fact, if someone were to start giving a description or explanation of such a thing would be like, or resemble, you'd know they were full of shit right away, because of course, if nothing were described it would not be nothing.
Another thing a little irksome about the question (really, its interpretation) is that it seems to ask for some kind of a reason, which would suggest a motivation. The intuitive interpretation of this question asks us to make the assumption that a basis of all possible types of realities must have a preference between two alternatives (it also asks us to assume that these two things are fundamentally alternatives). This is the reason that some people use this question to justify their belief that the universe was created by God. If nothingness is more 'likely' or 'preferable' than somethingness, and yet we exist, there must have been a type of 'choice' event in deciding whether or not there should be something (probably with the famed 'Nothing-Coin'), and that choice landed on somethingness (easy choice, really - somethingness is way more awesome).
But let's really turn the tables and ask that question. How could there possibly be nothing, what on earth would that look like, and why should we think that this type of universe is somehow... likely?
Well, I think a fair word to describe this universe would be featureless. In fact, it would have to have this property of featurelessness absolutely everywhere, and this featurelessness property would have to be exactly identical everywhere. And of course, we couldn't have things outside of this featurelessness, so it would have to be infinite. In fact, it starts to seem like a whole lot of work to create a universe that has absolutely nothing!
Whatever this nothing is, we must at least agree that it is uniform. Think of the amount of information required to describe a universe that has absolutely no features, no deviations from a this complete uniformity. Why should such a universe come to be? Why would every point in it to be exactly so, and not any other way, say five feet to the left (to borrow from Leibniz)? It seems like an incredible amount of storage space to store the exact locations of an infinite amount of nothing, all arranged perfectly uniformly throughout an infinite amount of space. In fact, since space is an emergent phenomenon that arises from the fact that neighboring bodies are relatively distinguishable, it would have to be so uniform that space could not arise, and it goes without saying, time. Imagine the amount of energy required to maintain this state.
And if it is hard for us to imagine how such a state could be created and maintained (or why) at our point in time, it was probably never any easier to create such a state at any time in the past. In fact, if you need creation, it is much much easier for me to believe that something was the thing that would create nothing than vice versa. Something seems very messy, very easy to create than nothing. 'Something' is like Mandelbrot's famously simple z(n+1) = z(n)^2 + c, which generates the (literally) infinitely complex fractal, while 'Nothing' would be like a list of coordinates for every point in the universe (which is of course self-defeating, because where would the coordinates be stored?).