They declined you because you don't have a website. Some decline you because you have a website but it doesn't get enough traffic. Some do it for other reasons.

Set up your website. Then approach them again. Work on getting organic traffic to it. In the meantime, put adsense on it.

Or promote another product.

Oh, You are already a member of Linkshare? If that's so, and the advertiser is someone you really want, then write them or call their head of affiliate program and ask them why they declined you. Sometimes they make mistakes and you can persuade them to take you on. Worth a try. Sorry I misunderstood.

I believe the networks look at your site before accepting you as an affiliate. That's because the merchants also look at your site before accepting you. I would advise you to put your site up and make it as professional and inviting and targeted as you can. It's understandable that they would do this because it's the same as choosing a rep for your product. They want to make sure that your niche and the credibility of the site match the product. Submit your site, and I'm pretty sure the networks will invite you in.