I'm just guessing at this point, but the Echo in its current state is more like a beta test for a select group of people (i.e. those who were invited).
There are so many features that people want and the Echo is capable of doing, but the engineers at Amazon have been dragging their feet in implementing them.
The Echo has been available since November 2014. It took 5 months for them to integrate Hue & Wemo to the voice interface. This is something that's pretty simple to do that even some hobbyists were able to "hack" their way to make it work. These hobbyists did it without any information or documentation from Amazon. Imagine how easy this would be for Amazon engineers? Yet they took forever to do it.
Once Amazon fulfills all of the invitation requests, I'm pretty sure they will start to branch out to retail stores. If a store currently sells the Kindle, they will most likely carry the Echo sometime in the future. By that time, I hope Amazon has implemented a full set of features that everyone can use.