You want to handle refunds, fraud, money laundering, chargebacks... That stuff is really hard, and if it's not your main line of business, you really don't want to do it. And if you don't have a payment processor do it, and you don't do it yourself, criminals will scam your customers and use your platform to commit fraud and launder money and you will get in hot water with the law.

Bitcoin can be used as currency for occasional transactions between individuals. But you don't want your business to depend on it. It doesn't scale, not due to technological reasons, but because of legal issues and customer experience expectation.