Because acoustic is factually the laziest most homosexual type of rock-like music; it's mostly unskilled faggots doing some limp, easy strumming and being all emo. Have you ever heard "Hey Delilah" or whatever the **** it is called? It epitomizes all that is wrong with 98% of acoustic guitar-based music.
Plus, it's a known fact that an electric guitar-based band immediately sells out when it does any of the following: releases a Christmas album, drops instrumentation for electronic BS, and plays/records with acoustic guitar.
John Cougar Mellencamp does it right, though. Extremely few artists are OK using acoustic.