Because this functionality sits squarely with Microsoft, and their Marketplace content pipeline is a massive and almost impossible to move behemoth. Given that it took them over a year to fix the bug with RBN1 showing up in the first place, can you guess our chances of getting them to add a feature to hide one specific type of content for one title until we mark it as published?
What I don't get is why you couldn't accomplish the same goals by making sure the downloads don't appear in the xbox.com marketplace until after they appear in the in-game store. I hope this approach was at least considered and the only reason it didn't happen was due to a lack of support for such a scheme in the xbox.com marketplace...
One thing that really surprised me (and the other creators) is how few demos are downloaded, relative to sales. We expected maybe 10 times as many demos as sales, but in fact it's usually the other way around. WHAT IS WRONG WITH YOU PEOPLE? DON'T YOU LIKE FREE THINGS?
The one thing that really gets me, is that if you get the demo from the website, play the demo, then buy it in game, that doesn't count as a website purchase. I'm willing to bet that a LOT of people do it this way, but it would be harder to track.