Some factors that might be force us not to go with Magento for Ecommerce are:
Takes Time to be Understood
Magento Developers Are Hard to Find
Community can Lead You to a Crossroad
It Doesn’t Serve the Smaller Businesses Well
Requires Great Hosting Environment
Magento allows you to easily integrate any 3rd party extensions, applications and additional plug-ins to optimise your e-commerce store, such as payment gateways, marketing automation, shipping providers and tag management tools.
You should be using Magento for E Commerce because
It's Open Source
It's Scalable
It's Easy to Manage
It's User-Friendly
Magento Supports Mobile Commerce
Search Engine Optimization Friendly
Security and Payment Options
Integration with Third Party Services
Analytics and Reporting
Community Support