i can tell you exactly why. and how to correct that.
first you are going to need to sign up for both Google and yahoo accounts
the next step is to add google webmaster tools to your google account
once logged in just copy and paste your URL to the box next to the add site button
once added you will need to varify your site the easyist way to do so is to upload a wile with the name specified by google.
finaly for you google setup will need a sitemap to let the google crawler see what links are part of you site and at what priorty to crawl them at. "if you need a sitemap file PM me with your domain name and i can make one for you that will work for both yahoo and google)