>Anyone have any idea why these oil rigs are taken to different places to be scrapped?

Optics. It's not a good look to have [insert oil / drilling rig company name here]'s rig being disassembled in places with no environmental or HSE controls.

As opposed to [insert oil company name here]'s tanker? Ok, they are sold to shell companies and renamed before their last voyage, but I guess with a little effort everyone can find the original owner...

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?

Turkey can take only a certain tonnage of vessels, anything which is above 10,000 LDT (Weight of ship in tonnes) is sent to the Sub-Continent. As ships grew bigger, ship recycling shifted from EU to Asia due to labor costs, ship size and environmental factors.