According to the A-theory (at least on the versions I am interested in here), it's an objective but temporally changing (and hence contingent) fact that the year 2013 is present (i.e., lit up by the spotlight, on the leading edge of the growing block, or the year containing all real events, depending on the version of the A-theory).
But why is 2013, instead of say 2012 or 10 billion BC, present? Note that our argument will work whether or not by "2013" we mean a rigid designator of a particular year or some sufficiently long definite description.
Well, the one answer I can think of leads to a regress: 2013 is present because last year 2012 was present. For then we ask why last year 2012 was present. And the answer is, presumably, that two years ago 2011 was present.
This regress is infinite or finite. If it's infinite, we have something unexplained—namely why all of these years happened when they did vis-a-vis the present, 2012 a year ago, 2011 two years ago, and so on.
If it's finite, we have something unexplained, namely why it was that N years ago it was the year 2013−N, where N is the age of the world.
Can we give a theistic answer? Let's suppose "2013−N" is a definite description like "the first year of time". So: Why was the first year of time N years ago? Answer: Because N years ago, God created time (or the universe). But now we have a new puzzle: Why did God create time N years ago (i.e., before the lit-up time, before the year containing all real events, or before the leading edge of the growing block) rather than, say, N−1 years ago? (Or maybe better: Why is it N years after God created time?) After all, this is a contingent fact. Last year, it wasn't true that God created time N years ago (i.e., before the lit-up time, etc.)—it was instead true that God created time N−1 years ago.
Let's try our regressive explanation again. For we could say that God created time N years ago, because a year ago it was the case that N−1 years ago God created time. But this regress is actuality a circularity. For if we apply this schema N times, we get to the explanation that N years ago it was the case that 0 years ago God created time. But that's just a fancy way of saying that N years ago God created time, which is what was to be explained. Oops.
Perhaps there is some other explanation. Maybe God created time N years ago because N years ago was the first year of time, and God could only create time in the first year of time. But now we have a new question: Why is it that the first year of time was N years ago? And we better not go down the road of saying that a year ago, the first year of time was N−1 years ago, as then we'll end up, in a finite number of steps, with the claim that N years ago, the first year of time was 0 years ago, which is just a fancy way of sayingt hat the first year of time was N years ago.
If I am right, then the A-theorist cannot explain why it's 2013. The B-theorist denies that there is any such objective fact, except for the trivial fact that at t0 it's 2013 which holds because of necessary truths about t0 and 2013.
Now perhaps a given presentist hasn't been convinced by the arguments for the Principle of Sufficient Reason. But nonetheless it counts against a theory that it posits contingent unexplained facts that a competitor does not.