[BubRoss]

I don't think anyone rejected it because of sandboxing. It is a new API that would only work on windows with that installed. Learning something completely new for a niche target is not very enticing. Also, if you can write something to be sandboxed, there is a good chance it can be a web page instead.

Sandboxing also needs to come from the other direction.

Having programs be made to be sandboxed defeats the purpose. What is needed is the ability to do contain all of a program's files in one place and isolate the access to the file system with permissions for other resources.