what is the main reason to use asp.net mvc instead of webforms?

Webforms is legacy (not obsolete) technology. MVC has been around for almost 2 decades and it has evolved far more. Webforms was created more than 20 years ago to help non-web developers do web through an event-based model and do RAD. Webforms will end its life with .NET Framework while MVC is included in .NET Core and later. To make it short, very few people keep doing Webforms which will make things harder for any newcomer.