- Familiar environment
Easy of use
Huge community
Interactivity
The ability to run a REPL prompt provides a great resource for testing and experimentation. Instead of having the edit-compile-upload cycle, it's basically edit the source .py file and run.
Comprehension
I think a lot of how libraries are used in the Arduino are pretty much used as black boxes... without a lot of comprehension as to their inner working or contents. For some, especially beginners, this can be a great way to build neat things with little effort. But for more advanced projects, a better understanding of your tools is necessary. Micropython's code is simple and clear throughout the code base.
Power Without Pain
Finally, I think the high-level nature of Micropython makes for easier implementations. It's heritage in version 3 of Python gives us a whole new set of tools to solve problems. This means it opens up whole new approaches to embedded programming.
So what can we do to encourage Micropython adoption in an Arduino world?