How did Sanchi survive but not Amaravati?
Various factors played role in keeping the stupa of sanchi intact at the place where it was built which did not exist for Amaravati:
1. Ruler of bhopal played a crucial role in protecting the ancient sites by providing funds for its reconstruction and studies. Whereas for Amaravati no one paid any attentions.
2. When Amaravati ws discovered no one knew the value of it and keeping it at the place where it was found. But luckily for Sanchi there were scholars who voiced against taking away of its parts.