I don't understand why ebay does not allow their users to accept bitcoin. (with their own responsibility)
Because they can't offer buyer protection with Bitcoin. When people get scammed (it's not a question of if, but when) they will be scammed while using Ebay's services. They probably don't want such tags. Even if they were to state in their TOS that they don't provide any guarantees or recommend that users pay with Bitcoin, people wouldn't care and would hold Ebay accountable.