Why is the metal tab of a BJT power transistor is always connected to its collector pin?
The electrical connection at the bottom of the diagram also makes a good thermal connection. Very important for a power transistor.

So, why can't you just reverse the Emitter and Collector in the diagram. You could, but it would be a lousy transistor.

https://en.wikipedia.org/wiki/Bipolar_junction_transistor#Structure


  The collector surrounds the emitter region, making it almost
  impossible for the electrons injected into the base region to escape
  without being collected, thus making the resulting value of α very
  close to unity, and so, giving the transistor a large β.