Short answer: most of they are not really interested in pay your product, but just like to visit your site. The rest of users maybe are really trying to buy.
You have two types of visitors who click to buy: that who want to buy, and that who will never buy (for this visit). Both are common, but you are interested on fist one.
For who is interested in buy your product, you should try to see the whys. Since your questions is just mentioning the payment method, few good motivations for interested clients are:
- Do not know how to use your payment method. Try to make it more simple, or give options
- Do not trust your payment method, or maybe your site who point to the payment method. Try to be more professional
- Some people tryed to pay, but they payment was refused and they leave your site. Try to give more payment options or take they e-mail for reminder for the future
Keep in mind that like in real world, people go inside shops, see products, but do not buy. On internet this is more common. Some e-commerces show that is less than 5-1% of the visitors that really will buy on your site.