As for now, Building Maker requires users to install the Google Earth plugin. Who knows whether that's only used for the visible preview of the building or for less obvious internal calculations? We don't know.
While Google Earth (and plugin) is developed by the Google Earth team and targets the desktop (or anything with graphics chip), Google Chrome is developed by another team that targets netbooks. They would have to join forces to do the port. Another point to consider is that you have to be sure about the architecture of your project before starting development. Maybe for ChromeOS (or any other new platform), a classic port wouldn't be ideal, maybe they are trying to push Native Client or WebGL forward or waiting until that technology is progressed far enough.