There are really two main reasons, but it's certainly something that could be reconsidered at this point.
- When we first updated App Inventor to support the newer Android themes, there was a bit of a backlash because the appearance of people's apps changed overnight. This led us to introduce the Classic theme as an option and we made it the default so people had the option of switching to the newer designs. At this point, we can implement an upgrader for existing projects to keep everyone happy.
- A lot of App Inventor curriculum exists from the last 10 years, and we didn't want to force people to use the new design when all of the existing published material uses the old style. This is still a potential issue without careful planning.