Probably because it's a bit of a difficult thing to explain to users in the first place. Instead, by providing just "primitives", they opened the doors to focused add-ons (like the one that isolates Facebook, effectively the first large-scale deployment that used it) without spending too much time on a feature that folks might have found difficult and might result in increased support issues. The status quo is that power-users can make it work, add-ons can use it if they wish, and the overwhelming majority of users can carry on happily ignoring its existence; not a terrible trade-off. But I agree that the management add-on could be better.

I believe Apple is doing something similar with their virtualization API - it's there, some folks can use it, but they don't want to surface it to the masses.