I always wondered how you go from a thriving city to an 'ancient ruin'. A lot of cities in India have been continuously occupied for over 2000 years (Kalyan, Ujjain, Bharuch, Patna). A lot of them don't have any obvious ruins from that era (most old buildings tend to be from the medieval era or rock cut caves/temples/monasteries that were forgotten about in later eras) compared to a place like Hampi or Angkor (although both cities were abandoned in the medieval era).

My guess is that for a city to have relatively well preserved ruins, it has to be completely abandoned due to some calamity. If a city is continuously occupied, old buildings get incrementally replaced by newer buildings, and very few old buildings get preserved.

Even with Rome, my theory checks out. We have a lot of Roman ruins because Rome was heavily depopulated after the Roman empire collapsed. A lot of buildings were probably abandoned and then buried by dirt over time (like the Roman Forum). The only other preserved ruins were buildings that were repurpose d (Agrippa's Pantheon, Colessium, aqueducts, etc).