It might be because of the high volatility, because merchants do not like high price fluctuations in currencies to operate. If they worked through a payment processors, they would have had to pay a additional fee, to get the bitcoins converted to cash. Fiverr has always been my favorite place to visit, when I had banner Ads made, but I have to look for something else now, because I always paid in bitcoin. ^hmmmm^
I am not fimiliar with Fiverr myself, so I can't say how they were accepting Bitcoin payments, but in general, the volatility shouldn't be a concern if merchants accept Bitcoin through BitPay for example. If you compare the business plan of BitPay that only takes 1% in fees, then it's still a far better and cheaper payment tool for merchants than Visa, MasterCard, PayPal, etc. But I am not sure what exactly their reason is for ditching Bitcoin as payment option. It could be that it wasn't being used as much as they were hoping for, and thus they thought it's not worth to continue supporting Bitcoin payments. Or it may have something to do with regulations. Either way, this offers chances to competitors to take over their Bitcoin users.