Ancient ruins

[Boyar Son]

I was wondering,why, the ruins are,well,ruins. (I know sounds kinda dumb). Why hasnt anyone made sure there still the way they were. The people lived in that city, then later it becomes a ghost town ( like palmyra, correct me if I'm wrong). could anyone tell me why people would let these fine building turn to to ruin and decay?

Also, I hear from my history teacher and t.v. on the history channel, that Alexander and his men saw the great pyramids, and said they were painted (or something like that). Did the Aegyptians long ago had paint on there sculptures and pyramids?

And please dont get this thread locked because of FLAMING.