Why? Preact is small, fast, and reasonably easy to use. But the main selling point for me was that, together with htm, I don't need any build system a la webpack. No node_modules folder. I just have to load one script (preact + htm bundled), define my components in plain javascript files and import them. Bliss for me.
[1] https://github.com/developit/htm
Edit: Just read that you want drag & drop style UI building, so this might not be for you.