My goal is to build a Python cross-platform app development tool, which uses Pythonista to provide cross-platform devs with better integration with iOS. Kivy doesn't seem to integrate well with iOS. For example, it does not look like I can embed a web browser on iOS using Kivy, and that's a necessary feature regardless of if I use it for UI or not, as I need to be able to load web content.
Speaking more generally on cross-platform GUI and WYSIWYG, I've actually contributed to wxPython and wxWidgets, and I can tell you, the workload required to maintain a cross-platform GUI toolkit AND get a solid WYSIWYG tool going is a ton of work, and you're constantly playing catch up as the native platforms add new APIs and functionality. While I wish the Kivy project luck, my past experience says that chances are I'm going to quickly run into roadblocks with support for various native things I want to do. Part of my goal with PyEverywhere is to allow cross-platform Python development in a way that avoids this sort of issue.
As for why HTML/JS/CSS, basically, since almost everyone uses a browser, browsers are the cross-platform GUI toolkits that get the most love by far, are highly performance optimized, and often have much of the device's native functionality exposed. Plus, there's lots of libraries and tools out there to help speed up development and add things like charts, games with sophisticated effects, etc. One of Python's big draws is the ability to pull in modules with lots of advanced functionality, and HTML/JS is the closest I've found to that on the UI side of things.
Anyway, sorry for any misunderstandings! I guess I conflated "developing cross-platform UIs in Pythonista" and "developing cross-platform UIs using Pythonista" in my head when responding. :)