Assuming the Atlanteans did exist (and I'm inclined to think that the story came from an actual now lost civilization) they don't exist anymore, so they're not as smart as us.
However, if you meant 'why were the Atlanteans smarter than us humans in the 21st Century?' then my personal view point is that they weren't, they were just an older civilization that had got further along the road of development - remember that the fossil record shows that as a species we haven't changed since we first showed up in any real way - if the story is to be believed then they certainly weren't any more advanced in behaviour since they managed to destroy themselves just like we seem bent on doing now.
Others would say that they had extra-terrestrial help, or that they were extraterrestrial themselves - that's a whole other matter.
We are in the 21st Century.
the 21st century
~this is the 21st century
Yes they do have batteries in the 21st century and had in the 20th century.
No, there is no apostrophe in the phrase "21st century."
is 21st century is an age of greed
The year 2010 is in the 21st century.
Significance of mutation in 21st century
characteristics of 21st century managers
AD 2015 was in the 21st century
The twenty-first century (21st or XXI) ranges from years 2001 to 2100, so until 2101 it is the 21st century.
The first year of the 21st century was 2001.