1. Amazon is still pushing out it's own ideas and features for it...they haven't fully released it to the wild west of developers (many have learned to wait until the dust settles before wasting time/energy on potential features that will be blocked or taken over by the powers-that-be).
2. The speaker bit is cool, but really it's just Siri in a speaker instead of a phone (very cool but not a "wow" out of the box)...it's the next gen. that I think everyone is really excited about and waiting for (when the SDK, and api access, is baked into more than just one stand alone speaker - that is, if/when it becomes the voice interaction for the internet of things; that becomes super exciting and interesting)
3. Not an adoption limitation, but I can say that having worked with the v1 of the tools for building on this stuff...it's still pretty clunky and painful. I think they (AWS) are adopting and upgrading that quickly...but there are some hurdles there as well before "anyone can build a voice powered/interactive thing" can become a reality (and that's the HUGE sensation IMHO)...
The Echo only does limited things and is locked to Amazon's ecosystem. You're paying $200 for an addon to Amazon Prime Music which is itself pretty mediocre compared to the other services in the field (Pandora, Spotify, Google All Access, Apple Music, etc). So really the only people who benefit from Echo are people with large MP3 collections they've uploaded to Amazon's Music Cloud, which itself is a niche thing to do now (unlimited streaming is "in" and MP3's are "out").
Plus Amazon Echo is a music appliance that doesn't focus on the actual sound quality. One speaker and one subwoofer, and even then they're muffled by the casing. So really you wind up with a bit of a gimmick. Voice recognition and a mediocre sound experience locked into Amazon's ecosystem for $200.
You can buy a $30 bluetooth speaker all over the place. The sound quality on them is mediocre too, but you've only paid $30 and at least they don't worsen the sound needlessly via the gimmicking round shape. The speakers as they are are unobstructed. So why pay $200 for a bad experience when you can pay $30?
At the same time, I don't think Amazon is the best at marketing as they have shown in the past with other devices. But I can't blame them with the echo wanting to start slow and gain traction and expand it out.
As to their feature set, I signed up for the dev/api access for it over 6 months ago but still have never heard anything back from Amazon. So if they want people to expand its feature set to make it better it would make sense to approve dev access a bit more timely. I have heard a few others tell me the same thing so I don't think it is just me.