According to Pistone, Galante was killed because he wasn't considered a legitimate boss which was holding back the zips from dealing more heroin. In order to continue their heroin pipeline, they had to side with the boss the commission recognized (Rastelli). See page 143:

http://books.google.com/books?id=fhBwVP1...%20&f=false

This is according to Pistone, so take it as you will. Does give credence to Hairy's theory, however.