The primary reason? Accountability. You'll be able to look at your traffic stats on your new site and see exactly what is working or not working. Does your Yahoo! Business Express listing pull traffic? Does it convert into sales? Does your Looksmart listing pull folks from MSN? On what keywords do THEY spend the most? Are your GOTO Overture folks converting better on the new landing pages? Do they call the new 800#? What about email addresses collected on the new site? Does marketing to them drive sales?
It is SO much easier to see results from a new marketing project when that project stands alone from your existing site. It's also easier to build a new site from scratch than it is to "fix" a site that's been around for a while, and we're not messing with anyone else's baby or design project.
After the initial project is completed, you'll have several directory listings, be well-positioned for the right keyword phrases in the most important search engines and directories, and have more traffic than you can shake a stick at.
(Orders? Sales? A lot of that is up to you, i.e. quality of your offering, brand names you carry, pricing strategies, etc., but I can help with conversion as well!)