If we look to a few weeks ago, where Steam (The PC game platform) cut off BTC support, the people at Valve justified this with two main reasons. The volatility of the value of Bitcoin and also the almost stupidly high transaction fees. On a place like Steam, where games can be from a few cents, this is ridiculous.

Places like eBay and Amazon see Bitcoin as a large issue in terms of logistics, as their systems are wired up for money - and implementing a solution for bitcoin may not be worth it for them in the long term. But hey, we might be surprised if they do start to support it, as there may be some advantages for them too.