It's part of establishing a brand for themselves. They want to be the face of the modern linux desktop. They've done a good job of this already with the tweaks they've made to GNOME (I run Pop_OS and I have had a very positive experience it), but if they really want to establish themselves, they need to create a branded experience all their own, à la ElementaryOS. A custom DE would go a long way towards providing that. It's also an opportunity to address and fix a lot of the UX mistakes other DEs have made, which is likely to generate lots of buzz (as evidenced by this thread).