Some reasons behind lack of eBay bitcoin adoption:
1. PayPal is owned by eBay - and both these companies are not here necessarily to make world a better place but to gain profits.
Introducing bitcoin payments on eBay would cut their profits rather substantially (sellers are paying fee: 2.9% + $0.30 per transaction).
2. There are more than 192 million PayPal's accounts active worldwide. Meanwhile bitcoin is used by less than 10 million people.
3. You need chargeback option on eBay. Bitcoin is not giving you that.
That summarizes it, Paypal was made for ebay, so anyone wanting to do business on ebay needed to own a Paypal account, that was how the millions of clients came about. Now that bitcoin is here and breaking borders, it present a threat to Paypal, so it is not something they are willing to consider in a rush though they are mindful of the potential bitcoin holds for ebay.
Not having the ability for chargebacks with bitcoin, is not a major issue, because a well structured escrow is well able to take care of that.