People who sell stuff online should definetly prefer bitcoin over fiat, since its much faster and safer. Bitcoin can't be charged back either.
Not entirely wrong but.. (here's a long post)
The number of transactions being made in Paypal in a day proves that a lot of people (buyer or seller) prefer using Paypal. In Paypal, you can simply connect your ATM card and then that's it. You can start paying for anything you want to buy on Amazon, eBay, etc. It's intuitive/user-friendly/noob-friendly so the majority of people like it. Thus, there are more buyers and sellers that use Paypal.
Since there are more sellers that accept Paypal, then that acts as one huge force to make someone who wants to buy something online use Paypal.
And since a lot of buyers are in Paypal, newbie sellers would prefer using Paypal too (regardless of its disadvantages). What they need is audience. All these chargebacks will be battled by them along the way but it shouldn't bother them in the start. It's not on their mind when they were signing up.
When it comes to bitcoins: It's decentralized, fast, and low-cost when it comes to transactions. But all these things do not bother a normal consumer. When they see that they're going to pay, like, a dollar or two for the transaction fee in Paypal, they wouldn't actually mind it. People don't care about these things that much, okay? People like to spend a lot of money when they're shopping most especially when they're buying online because people want to act as if they're rich (even if some aren't) so transaction fees wouldn't bother them.
(so that's my explanation why a basic consumer would prefer Paypal and why sellers should prefer Paypal)
The only people that sell stuffs that will prefer using bitcoins are the ones that sell bitcoin-related stuffs (i.e. physical bitcoins, bitcoin collectibles, etc) since their buyers would prefer buying using their bitcoins (since they have it).