The most obvious difference is that OpenCart is free to use, whereas Shopify and BigCommerce are paid solutions. All three solutions are easy to use, but only OpenCart gives you the ability to customize its source code.
Unless you can find an extension, adding customizations to Shopify or BigCommerce will either cost you a lot, or just be impossible. Even though Shopify is currently the most prevalent ecommerce solution, OpenCart is slowly starting to catch up.