I'm a very happy PayPal user and have been known to abort a purchase if I get to the payment screen and find they dont take it. That would mean getting off my butt, finding a debit card and several more clicks away from completing the purchase.
Paypal means one card registered to one company through whom all payments are made. They show on your statement as Payal - name of retailer. My card isnt then registered with all the different companies who you might only use the once.
From the other side, retailers pay a fee for taking credit/debit cards as they do Paypal transactions. Its as broad as its long for them, its just that some dont have enough transactions to bother I guess.