Because Chittagong has no facilities for breaking oil rigs. When they break ships, they run them at full speed toward the shore to beach them. Can't do that with a rig.
You haven't not seen the bottom part of a rig then. In Alang, on high tide, the rigs are beached as close to the shoreline as possible. Please don't provide misinformation with assumptions.
Can you run a rig full speed onto a beach, like they do with the ships?