I think a few of those devices got a lot of publicity, especially when they were almost immediately sold at steep discounts, but:
1) The UI (IMO, at least, the smaller the screen, the more of an advantage iOS and Android have over Metro in terms of usability.)
2) Launch problem discrediting the concept (the Stream 7 was one of several tablets using a new Intel Wifi card that had bad, bad, bad driver issues. Do I want another Intel-powered tablet? Yes, but that's because I'm an Intel fanboi, not a non-techie consumer who's bad at statistics, making a buying decision.)
3) Hesitant manufacturers. The last Windows Tablet models made approximately none of the monies.
4) Momentum: go to Amazon and filter for <7.9" tablets:
There are 153 Windows products - the most popular one appears to be the Stream 7, with 600+ Reviews.
There are 2,011 Android products - the most popular one appears to be the Galaxy Tab 4. Twice the price of the Stream 7, over 5,000 reviews.
There are 47 Apple products - the iPad Minis appear most popular. Twice the price of the Galaxy Tab 4, over 5,000 reviews.
You do the math.