That's not the only reason. The other reason is that although it's pretty obvious to any one person how it should work, no two people's ideas are the same. Depending on how you are using containers, the answer to a number of questions can be completely opposite to someone else's usage. The theory was that Firefox would provide the underlying functionality, and specialized addons would specialize it to a style of usage.

In practice, it feels like that has hurt adoption. You can't pave the cowpaths if the cows aren't venturing out of the corner of the field.