I think it's because he's one of the last people to really shake our understanding of reality. While there were other contributors to relativity, it was largely his creation.

By contrast, there were dozens of brilliant minds involved in developing quantum mechanics.

Also, "Einstein" has a better ring to it than "Heisenberg" (which I didn't even spell right on my first attempt).

Undisciplined

It's interesting that somehow Einstein ended up being the go to any definition of "smart person".

Of course Einstein is a powerhouse and a great mind, but even that mind struggled with quantum entanglement, calling it "spooky action at a distance". He found that idea troubling and counterintuitive.

We have since got more and more brilliant minds all around the world, but no one else has been crowned as "the new Einstein" in a way. Before him there was Newton I guess, but that's not used today as a "smart" person anyway. 

It's always Einstein. Not sure why, I personally find Richard Feynman a more interesting character and a brilliant mind.