In Webforms:
Both Viewstate and Postbacks have been made lot of problems and increased complexity of the web application development. Many web pages having hundreds of KB size of Viewstate that affected the performance of the applications sometime.
Developers do not have the control of the rendering HTML of web forms and Server controls that render html with mixed inline style and deprecated tags that does not follows standards.
The page life cycle of the Web Form is too complex and has the tightly coupling between all things in the ASP.net framework and a single class is used both to display output and handles user input.
Unit testing is almost an impossible task. Today unit testing is very important in modern software development especially when we following agile methodologies and practices. Since web is a stateless thing, Events, Postbacks and Viewstate are not a good way.
With asp.net MVC all these things are simplified
If these things don't apply to you and you enjoy using Webforms then stick with what you do best. Don't try to fix something thats not broken.
For more detail refer to : Shiju's blog of ASP.net MVC Vs ASP.net Web Form
