A couple of years ago we built an e-commerce website for Caroline McGrath based on Zen Cart. While a great CMS in many ways, it always seemed bulky to extend and customise. Eventually it meant the Zen Cart version got patched, but no longer upgraded to the latest version.
Having built a few other e-commerce projects with WordPress and the WooCommerce plugin, it seemed like a good fit and the whole website with thousands of orders and customers got migrated, as well as the design.
With the move to WordPress and WooCommerce, it became a lot easier to build new functionality and the pool of free and commercial plugins to use (no need to always reinvent the wheel!) is a lot larger.
The client now spends less time on navigating the admin and more time on adding new great home accessories to their shop.