The readme on their github[1] says that Live Writer was originally part of an acquisition. I would guess that winforms may have already been in use when they acquired Onfolio. There may be other reasons as well, but I don't know what they are :)
Quote from Scott Hansleman's blog post[1]
"Much of the code in Open Live Writer is nearly 10 years old. The coding conventions, styles, and idioms are circa .NET 1.0 and .NET 1.1."