There could be lots of reasons why.
Is your SEO targeting the wrong keywords, and people are not finding what they are looking for? Have you set up your Google Ads properly? Are you buying 'site visitors' from a low-quality source? Is your Analytics set up poorly and tracking a lot of SPAM and bot traffic?
While all the above are possible, often the most likely explanation is your site has yet to be optimised for conversions. A site can look great, but if it is not designed to convert then people are unlikely to buy?
Here at EthicallyMAD, we are all about conversions; turning qualified visitors into buyers. Give us a call, and we will help your site achieve your goals.