I hadn't heard of it until now, but my first guess would be because it's eBay-branded. If you're looking for a way to set up your online store, why not go for the technology created by one of the biggest online stores ever?
The main reason is the scalability, functionality and support. For example, you can customize the the deepest level, such as product page, pricing, each and everything which is not allowed in other platforms. Tons of Mag developers are there to offers free help in different forums. Magento goes beyond reach, thats why people choose it.