It's not that popular online. Like as in online shops. Just try and see how many online shops that accept bitcoin you can find. I mean the list is growing by the day but still bitcoin payments are still in their infancy. But still, from what I can see things are looking up, slowly but surely.
I am sure that there are a lot merchants willing to accept Bitcoin as optional payment form, alongside the plenty of others they are working with, but most of them simply don't know Bitcoin, or they get distracted by the volatility. I am sure if they realize that they aren't exposed to the volatility if they make use of a payment processor such as BitPay, that they will definitely be open to accept it as payment form. It's a lack of understanding that plays an important role here. If I was a merchant that didn't know much about Bitcoin, then sure, the volatility will be a solid reason for me to not allow Bitcoin payments. But if I know we have third party payment processors that take care of everything, then heck, why not? I have nothing to lose, and the same goes up for others.