The conversion rate at a website depends to some extent on the user interface. A well funded startup selling conventional products, can use any of the shopping carts available, but if the billing method is different, a team of programmers are needed for developing a suitable website.
Finding a good programmer can be a difficult task, especially if you do not have a programming background or a large budget. A lot of testing is required to remove any bugs before the new interface can be used, which is time consuming. The checkout process is comparatively difficult to debug as payments have to be made. A new startup may not be able to afford this, but this can affect sales and customer experience.