Danny Sullivan has a good post, "Why Search Sucks & You Won't Fix It The Way You Think", that looks at the lack of innovation in search user interfaces and several failed attempts to improve them.
Danny does not get into details about why the attempts at improvements did not succeed, but I think it comes down to too much work.
Type stuff in a box, punch a button. That's easy and it works most of the time.
To be an improvement, any substantial change in that UI paradigm will have to be clearly easier to use or produce substantially better results almost all the time. Probably both. That's a high bar.
Type stuff, push a button. It's going to be hard to beat.