Probably because the tutorials for beginners reach the most number of people (because advanced users are more likely to self-learn or use documentation, and because advanced topics are often specialized). Therefore there is the most value in making a good beginners tutorial.

Also, a tutorial for beginners can be written by someone who isn't an expert, but is a full or part-time professional writer or communicator. A tutorial for experts must be written by other experts, who don't have the time to learn and practice communications skills as much.