Because Paul Halmos was an exceptionally good writer. (I greatly enjoyed his "automathography" called "I want to be a mathematician", though it will probably appeal significantly less to people who aren't mathematicians themselves. See also his article "How to write mathematics" at https://www.math.toronto.edu/mgsa/assets/teaching-seminar/ho....)

The average Wikipedia contributor (even ignoring the possibility that they might be a robot) is not an exceptionally good writer, because very few people are exceptionally good writers, because that's what "exceptionally" means.