There is an extremely easy answer to this, which Busta said:
BitCoins are a hassle to acquire for 97% of buyers. It's not as simple as using your CC / Debit. You must find a legit exchanger, then send them a complex pay method which is usually just as safe as BitCoins anyways. BitCoin's primary advantage is the anonymity they provide. There are already other methods which are essentially chargeback free (LR, WU, etc.).