My guess is that that they’re not set up to take them, since they’ve dealt only with wholesale customers until relatively recently, and probably their business is still primarily wholesale. Their wholesale customers are probably on net 30 terms or something similar. So to take credit cards they would have to get a merchant credit card account and/or a secure payment processing service…they might also need to add other security (encryption, etc.) to their website as well. Its probably easier for Windstone to just use Paypal instead of doing all that.
It might not make sense for Windstone to take credit cards directly until the retail side of the business becomes more substantial. But it definitely would be more convenient for some customers if they did that! 🙂