Well, it seems there are three main reasons.
Web technology is still new and the way marketers take measure varies from one agency to the next.
So if the same 500 people are each accessing your site a hundred times a day, advertisers will count 500 unique visitors, and disregard your 50,000 hit count. Apparently business is very brisk. So, while perhaps you've got a way cool site with great numbers, an advertiser still may want to wait until you've been around for a while. If your business plan relies on incoming advertising dollars, perhaps it's wise to think in terms of years, and not months, to achieve your objectives. This leads to a fourth challenge that, given the anonymous nature of the medium, is a tough one.