Yahoo has the right to terminate their search deal with Microsft, "if the trailing 12-month average of the (revenue per search) in the United States (the "U.S. RPS") of Yahoo and Microsoft's combined queries falls below a specified percentage of Google Inc.'s ("Google") estimated RPS measured on a comparable basis or if the combined Yahoo! and Microsoft query market share in the United States falls below a specified percentage," according to a document filed with the U.S. Securities and Exchange Commission.
In addition, Yahoo can walk away, the document says, if Yahoo’s and Microsoft’s combined share of search queries falls below a certain percentage of the market that the two sides have agreed on but not disclosed.
Maybe because they've tried and failed to come up with a search engine (and the appliance world to support it) which works for the 21st century.
See Yahoo's experience with Inktomin, Altavista, and quite frankly even BING.
Google owns the net's version of "paved roads and wheels" and Yahoo ain't a likely contender to shift that paradigm in the least.
Yahoo, according to the filing, may terminate the search agreement if the "trailing 12-month average of the RPS (revenue per search query) of Yahoo and Microsoft combined queries falls below a specified percentage of Google's estimated RPS measured on a comparable basis or if the combined Yahoo and Microsoft query market share in the United States falls below a specified percentage."
It appears Yahoo engaged in some savvy negotiating, N. Venkat Venkatraman, a business professor at Boston University, told the E-Commerce Times. "It is compelling Microsoft to meet -- and beat -- not only Yahoo's trailing 12 month performance in terms of Revenue Per Search but also Google's estimated RPS."
Yahoo can also terminate if Microsoft attempts to exit the algorithmic search or search monetization markets.