We're not necessarily smarter (native intellect) but we are a shitload more enlightened (educated, experienced). In Plato's time, only the absolute social elite could converse in this manner, whereas now we have much broader and deeper knowledge available (and discussed) on pretty much every aspect of life that is common between our times.
Don't fall into the trap of deifying past thinkers; they were humans too. Yes, we can learn lessons from them, but our populace as a whole is more enlightened than the populace as a whole in Plato's time. One simple example is egalitarianism; while not perfect now, Greek democracies were only democratic for elite men. Sucks to be you if you're a slave or a woman because... um... that's just the way it is? If you're going to be making theory, you should work from solid foundations - the ancient Greeks had some really wrong ideas about the role of the physical heart and brain of a human, and no-one today would talk about the left ventricle being the source of human heat, for example.