The main reason for which one should choose Magento opposed to other e-commerce solutions is that it's the best built solution, and its excellent architecture allows for great customization while maintaining a robust and stable platform.

"Magento is going to win the e-commerce software war, hands down. I've never seen another e-commerce platform so well architected and well designed as Magento." Jack Aboutboul, RedHat

"Magento is a robust e-commerce software application designed to be simple to customize." Inc.

Picking a well built e-commerce solution is important as one doesn't want to be locked into a solution that has no future, or a solution that is constantly failing or is impossible to extend with the features required by the business. For this reason there is a very noticeable migration of existing stores to Magento