(Perception of) better results: until proven otherwise - why would you use an alternative/inferior SE - unless you value other things like privacy more than the content of the results - this doesn't apply to most people
Fast/it just works: give me a compelling reason to switch. "Privacy" really isn't enough for most people.
It's default: the default FF SE? The default homepage for a bunch of people? Default search in Chrome? Check. Check. Check
Network effect: Remember the expression "Nobody ever got fired for choosing IBM" (that might be too old for a lot of the readers here) - but the same thing applies - "nobody ever got in trouble for choosing Google"