Why is battery being drain after new alternator put on?

What else is in the system? Was the battery charged and tested when the new alternator was put in? A new alternator can blow out if it is trying to charge a "Bad" battery. Was it a new alternator that was tested by a shop? It is possible it is a factory defect. If it was done by a shop I would take it back to the shop.

It is also possible that there is a wire off or grounding that is draining the system. Have you installed a stereo amplifier? Some amplifiers draw more current than the car can provide.