@Sander
1. Control
Look at human history for the biggest example. Those with the biggest beat sticks usually ended up being dominant while others ended up with pinata status. Humanity has always had a nasty competitive streak which only gets worse as populations grow. So imagine if any sort of government with reasonable access to resources were able to take advantage of lost tech freely?
2. Credit
If lost technology did influence modern technology then people would maybe atmost deserve half-credit for reverse engineering shit. All of a sudden, we would not be as smart and innovative as we had thought ourselves to be. Religion, how we developed as a species, heck, history itself would have been one big fabrication.
I am playing a bit of devils advocate here but there are good reasons why governments would like to control information. Otherwise we wouldn't have a problems with espionage and things like Operation paperclip would have been un-necessary.
Most likely however Atlantis, if it existed, was just another contemporary world power that ended up fadinng into obscurity like thier brethren.