Modern Retail has noticed a trend over the past couple of years where websites are forgoing the navigation on the product page in favor of using the entire page to display the product.
The main reason I've heard for this approach is so the Add to Bag button can be "above the fold." While designing for above the fold was important years ago, it is not nearly as important today as people freely scroll up and down, especially on tablets where it is a simple flick of the finger.
Instead, Modern Retail often recommends keeping the left-side navigation in place so visitors can quickly and easily move throughout the website. Yes, breadcrumbs can be added to the top of the page to aid in the navigation, but Modern Retail believes it shouldn't replace the main navigation.
This approach avoids forcing the visitor to click on the top-most menu to go to another section of the website, which many people find annoying because it requires too many clicks to navigate the website.