I'm guessing that Amazon has so many other things going on that releasing the Echo in other countries is just too much to do right now. Things like Prime Now, the orange pickup/dropoff lockers, drone deliveries (if they're still planning to do that), Amazon Fresh, Dash Button, Prime Music/Movies, and a slew of other projects.
Echo support is its own department, separate from all other departments at Amazon. If Echo were to be released in another country, the same would be expected. Bugs and issues that may be a problem in the US might not be the same in another country. A simple example would be the spelling and definition of words (US color vs. UK colour, US fanny is an innocent word vs. UK fanny is a vulgar word). That would mean different versions of the Echo software, each customized to the particular country it serves. Understanding spoken words could be a problem as British accents might be difficult for Alexa to understand. Perhaps Amazon needs to build from nearly scratch for all of these differences and issues, and that's why it's taking so long.