Because it only gives you screenshots, not actual webpages you can click. That might help find glaring CSS bugs, but is relatively useless otherwise.

I've never heard of it, or seen it referenced on any design blog, or seen any marketing of it whatsoever. I'm not necessarily claiming that I'm an accurate sample of the web development/design community, but it certainly seems to be pretty under represented.

From what I can tell from it's product page, it seems to have a very gimmicky marketing-speak write up (count the number of times the product is named!). It also doesn't let you run IE7 and IE8 at the same time.

I use this product, at times.

I think it's an example of a good intention on Microsoft's part that hasn't been executed or marketed incredibly well.

Any web developer worth salt has to preview their work on IE6-IE9. And everyone has to jump through hoops to make this work (And no, VMs are not an acceptable solution. Why should I bog down my machine with a VM when I just need an extra freaking browser?)

So there comes Microsoft Expression Web SuperPreview. How did they even come up with such a name? What it does is it allows you to open several frames side-by-side, each with a version of IE. It's a great idea, it's JUST what I needed. But what it doesn't let you do is actually interact with the pages. You can only see, not click. Want to test a JavaScript behavior? Sorry.

The gimmicky name and the lack of marketing evidently obscure it further. No surprise you've never heard of it. I completely accidentally stumbled across it last year.

Edit: Link with screenshots: http://expression.microsoft.com/en-us/dd565874.aspx