There have been plenty of external enclosures demoed at different conventions, the last years.
Many of them have been presented as "coming soon".
But they don't come.
Now why is that?
The answer I've read up on is that Intel blocks them.
They refuse to give them a TB certification, and if they don't get that its much harder to sell them.
Also TB1 have been a bit to slow for eGPU to make some real difference.
TB2 is faster, but its still not really as fast as one would like.
How ever i personally feel this could be the future, and TB2 is a great stepping stone for giving it a market. If it would become a popular product on TB2, newer ports would soon come that works better for the application.
So if Intel just stops with the licencing fees and the restrictions to TB, it could really take off and become a great standard. But until they do that, TB will keep being a niche standard, and will have a really hard time "taking off".