First of all, just being open-source doesn't make Windows Forms or WPF cross-platform and in fact, that's specifically not the goal or focus here.

Secondly, Paint.NET includes quite a bit of non-WinForms/WPF code by now, talking directly to DirectWrite and Direct2D in places and probably other things as well. It's very much a Windows-only application and porting it would require rewriting a lot of important core code of the application.