While companies like Paypal have offered businesses an alternative way to accept payment for goods/services, they are not what most businesses use as the primary method to accept credit cards. 

There are two reasons for this.  First, you will lose a lot of business if you only offer Paypal, simply because Paypal makes it very difficult for the average shopper to pay without a Paypal account. What happens is the person goes to checkout. Now if the person does not have a Paypal account, yet they see a large Paypal button (with no other choices), you will lose a lot of customers there.  Additionally, when they are on the payment page, Paypal makes it difficult for people to see that they can indeed checkout using a credit card.  They do this to try to convert more people to using this service.

The exception to this is if you are using Paypal as a merchant account + gateway.  Some shopping carts, and companies they have built customized software, are integrated seamlessly with Paypal to process credit cards for your online store.  However, that brings us to the second, third, and fourth reasons why established companies rarely use Paypal: 

• Paypal does not have business class support. Getting answers to non-routine questions can take weeks, if at all.
• Paypal is not cheap.  Do you really need more reason than this?
• Paypal is known to lock up company funds for months at a time, with no way of unlocking them.  Paypal has locked our funds before. This is not an uncommon occurance.

In short, a true merchant account and gateway are going to be easier, and much better as a main payment option.  Paypal can sometimes make a good second alternative if your customers demand it.