These are reasons that why WPF is better than Winforms:

A->Any where Execution(XAML) : Any where Execution means you can create your UI once in WPF and then run it as window application or run it as web application. This is possible because internal representation of WPF is XML then you can run your application as window application or web application.

B->Binding : Binding means object communicate with each other. Object can send data or receive data with each other. Object communicate each other simply by Binding set to each other.

C->Common look and feel(styles) : If you want to set common look and feel for buttons , textbox etc you can set common look and feel . You can create style for common look and feel and applying in forms.

D->Declarative Programming : Declarative Programming means you described in abstract manner how your code looks like rather than writing actual code. You did not need to write much line of code for setting binding, styles etc. It makes development faster because you write less line of code.

E->Expresion blend and Animation : WPF uses Directx. Directx is a nice framework for doing animation. WPF can do animation because directx can do. By using expression blend we create animation and use animation.

F->Faster Exceution (Hardware Rendering) : WPF internally uses Directx and winfroms use GDI or GDI+. Directx has capability to do hardware rendering while GDI does only software rendering.So WPF uses GPU rendering means render display faster.

G->Graphic Independency(DIP) : Graphic Independency means resolution independency. Resolution means total no of pixels that fit on your screen. WPF does not use pixels as a measurement unit. It use DIP device independent pixels.1 DIP=1/96th of the inch means if you change your resolution it does not matter. It looks same.