One answer to the question of why Relevance Feedback isn't available is that it really is available, it's just not prominent. In truth, there are some systems available that use Relevance Feedback. The Fusion project [48] is a meta-search engine similar to MetaCrawler [47]. It forwards queries to six Web search services and collates the results. Currently it is prototyping a Relevance Feedback system. Euroferret [29] based on the Muscat system [32] uses Relevance Feedback to aid the user in expanding queries with appropriate terms. Alta Vista [8] LiveTopics is a Relevance Feedback technique where the system presents the user with a choice of words and allows the user to expand the query based on those words directly. Deviating from most systems, LiveTopics doesn't get relevance judgments from the user initially, and thus presents all candidate terms to the user. Excite [14] has a similar but more rudimentary feature as well.
A similar answer to the above is that while Relevance Feedback is not present on many systems, it is coming. The systems on Alta Vista and Excite would indicate that they are moving towards inclusion of Relevance Feedback. Since most of the popular Web search systems available were created from scratch and are only a few years old, this is a perfectly plausible explanation.