That would explain the creation of C#. From there, it's not such a weird decision to aim for a single platform bringing together all the "MS languages."
It depends on the domain in which we are speaking. If I was programming for Windows and only Windows, really there is no reason to NOT use MS's development languages (VS.NET).
One last note is that Swing apps aren't slow if you code them correctly. And SWT is just plain ugly and has been way overhyped due to the success of Eclipse.
Eclipse is an advanced IDE (but an average editor) which uses a
non-Swing gui toolkit called SWT