Just speculating, but it could be a couple things:

Providing more choices can often result in consumers simply giving up. At a former job we did an A/B test where we showed some visitors two options to buy our product and added a third option for other visitors. In the three option scenario, the bounce rate was about 10% higher.

Or, it could be that there are folks for whom bitcoin is linked with drugs, gambling, piracy, etc. Maybe seeing a bitcoin option makes the site look less reputable.