...or better say, because avast! is not multimedia kind-of-application, so it's not that attractive for skinning... besides, let's be realistic, not so many people knows about avast! You would be suprised how many people never ever even heard about it nor they have any clue it's an antivirus. Norton and similar antiviruses has unbelievable propaganda in here, and I am pretty sure similar situation is in USA too.
WinAMP, WMP, BS Player and some other multimedia tools are unbelievable supported with skins, but that's just because millions of people use those programs, and when they use them, they are most of the time present on their screens. That's not situation with avast!. Most people, just keep avast! in their system tray and relly on resident protection. I am pretty sure some never even opened avast! GUI. And also, we are witnesses how many people actually complained about avast! looking silly with all those skins... although I can't understand that... how one antivirus or any other utility can possibly look silly if it has MAC OSX styled skin, or KDE styled skin or any other of those skins that mimics some OS.