why was mohejo-daro extinct?

Answer 1

I guess you are asking about Mohenjodaro. I remember reading in some history books that though the exact reason was not known the historians think there are two most probable reasons.
1. They were wiped out by the foreign tribes like the Aryans or someone from Afghanisthan or Persia.
2. The Indus river had changed its course (quite strange to hear) which had forced the to move away from the place.

Answer 2

Everyone went away from there.