Understanding history (or failing to)

Something has been puzzling sociologists and criminologists over the past decade or so:  why are crime rates dropping so dramatically?  A couple of days ago Free Exchange, over at Economist.com, had yet another explanation: the advent of antidepressants and anti-ADHD meds.

This one seems weaker than most (as time goes by, the efficacy of both these drug types seems to be dropping:  it's possible that their action is completely misunderstood), but that's not what I'm interested in.  It's this:

This social change is going on, right now, in front of our eyes.  We have more statistical, analytical, and data-gathering power than anyone ever has, and we still can't understand what's causing it.  So why do we think we can explain anything about (say) how Goths, Alans, and Vandals overwhelmed the western Roman Empire?  Or how Western Europe managed to dominate the global economy in the 19th and 20th centuries?  Or anything at all?

Don't get me wrong.  I love history, and will continue to read it.  I just don't know that I can ever believe any historian's statement that "this happened because of that".

Or a politician's statement about the same.  If we'd passed dramatic gun-control legislation in the early 90s, many people would be saying that, clearly, the drop in crime rate was due to those restrictions.  And how could anyone argue?  It would be absolutely obvious.

Disclosure:  I favored gun-control legislation back then, and would certainly have reached that (completely incorrect) conclusion.  I no longer think guns are a particular social problem (partially because crime rate is clearly not correlated with them), and think that we should take the Bill of Rights seriously, even though I do not own a gun, and never plan to.