Because they aren't really improving WinForms all that much (which might actually be a good thing). Depending on who you ask, people will often claim WinForms is an "obsolete" technology.
Whatever. I know I will not use WPF because WPF not multiplatform, and I am not interested in writing software that is hopelessly tied to the technology of a what is essentially a large competitor. Unless of course Microsoft decides to be charitable and donate the WPF source code to the Mono project.
If we let obsolescence and lack of support/updates put us off we'd be changing UI frameworks every couple of years!