I'd say that the #1 reason for their decline was the fact they they've been seen as competing with Google for the search crown. Maybe I'm wrong but maybe Yahoo should have been focusing on (what I consider) their strengths: delicious, flickr, y!live, fireeagle, developer network, etc.
Competing with Google in a field where Google excels, without trying something VISIBLY/OBVIOUSLY different/better was a losing strategy.
Competing with Google in a field where Google excels, without trying something VISIBLY/OBVIOUSLY different/better was a losing strategy.