Because search engine spiders cannot crawl pages that use frames. A framed Web page consists of a home page (or frameset), which contains HTML code that defines and links to the frames themselves. When visiting a page that is using frames a spider only sees the HTML code on the frameset page; not the primary page body contents, which are located in the frame elements. Thus, visiting spiders do not have any actual page content to work with and the page consequently is unlikely to receive an opportune search engine ranking.
An alternative to omitting frames is to add a tag to the master frameset page. The tag enables you to create an alternate no-frame version of a framed Web page, and when a search engine or browser that cannot read frames comes across a framed site, it will instead read or display the contents of this tag.