Because they're still tweaking it. Personally, I'm happy to see beta versions called a beta. Half the stuff out there in boxes on retail shelves should still be marked "beta" if you ask me. Take your time and get it right, Google!
some study shows that the quality of news.google.com matches the one of cnn and other news sites. google hasn't figured out how to make money off of its news site yet.
They have made more changes in the past several weeks, showing that they are indeed still tweaking it. Some of the results are occasionally way off, though -- maybe that's why they haven't removed the "beta" designation...
I had read in an interview/article that Google will continue to call it a Beta until such time as they decide that it will become one of their standard products (sic when it makes money). If they decide to drop it, it is not a failure. They just decide to not offer it as one of their revenue producing products.
The slant I've heard on it is that google puts its beta products out for the public to try. Some companies put their beta products out there for the public to buy.