Scottie Claiborne provides a good explanation why anchor text trumps alt tags:
General theory holds that alt text isn’t as “powerful” as link text, and for good reason. Webmasters who try to “game the system” stuff zillions of phrases into their image alts in an attempt to rank better for those terms. Because it isn’t an element that is likely to be seen by users (unless they hover over an image), it can be more easily abused. In general, search engines prefer to give more weight to elements that users can see.