Google started encrypting searches for users who are logged into their Google account while searching last year. As a result the keyword that the visitors search to arrive to your site is not passed in the referring URL. This change affects all analytics providers, not just Google Analytics, moving to another program will change nothing.
Google posted a blog on the subject "Making search more secure: Accessing search query data in Google Analytics"
When a signed in user visits your site from an organic Google search,
all web analytics services, including Google Analytics, will continue
to recognize the visit as Google “organic” search, but will no longer
report the query terms that the user searched on to reach your site.
Keep in mind that the change will affect only a minority of your
traffic. You will continue to see aggregate query data with no change,
including visits from users who aren’t signed in and visits from
Search Engine Land has a good rundown, including an interview with Matt Cutts on the subject:-
In Google’s new system, referrer data will be blocked. This means site
owners will begin to lose valuable data that they depend on, to
understand how their sites are found through Google. They’ll still be
able to tell that someone came from a Google search. They won’t,
however, know what that search was.