Because I choose to disable it by default for security and privacy reasons.

There is no good reason that this site should require javascript. Nothing it does is improved by the javascript requirement.

There is still a large number of mobile browsers whose javascript support is minimal or non-existant. Sure, the iPhone/Android market will be OK, but what about the Blackberry, Nokia and Motorola folks who still make up a majority of mobile browsers?

Also, a good number of security conscious folks who run Firefox also run noscript. This means that they can't use the site without explicitly whitelisting the site's javascript, which reduces it's immediate usability.

I use No(t)Script to whitelist sites I trust. Noticing there was blocked JavaScript on a page of simple text would prejudice me against it.

Because using code plus data where just data can be used introduces security risks and limits what people can do with the document. I would have hoped that the industry would have learned that already from PostScript, PDF, COM and the Microsoft Office formats.

Usability. I use a JS-capable browser, but many blind people don't, and they may be interested in this tool nonetheless.