Because that's not how requirements docs. are written. We have to go with the situation as it is rather than as we would like it. Buy in is going to be the biggest hurdle.
Maybe (probably?) i'm in the ignorant minority, but i would disagree and posit that email is actually how requirements docs are written. in my experience, very rarely are the client and developer ever in the same town, let alone the same room. in my day job, i consider myself a "small web shop" of 1 man, albeit working for a company of thousands (meaning all my clients are in-house), and only once in the past year have i actually been able to sit down with a user and go over requirements. in every case, the application is spec'ed and changed and fleshed out with a flurry of emails. unfortunately, the same holds true in my freelance gigs. in this regard, an email/web-based solution would be fabulous, and ultimately more useful and with a broader appeal, i think. When faced with building a web application, which is by nature a rapid development process, nobody wants to create Word documents. To most people they seem like a beaureaucratic artifact.

Again, i'm probably just woefully and detrimentally ignorant.

"Buy-in" certainly is going to be the biggest hurdle. I would bet most small web shops, the type that could benefit most from your project, don't even use requirements documents. They operate, for better and certainly worse, just as I explained above.