John Hawkinson had set aside 4.2/16 ("under the label "Numerology" since he had the wisdom to see that the numbers in themselves could be valuable").
My opinion is that among the reasons it is so well known are that it was designed from the very beginning to be memorable, and they folks setting up this service had the foresight to realize that having it on an easy IP was valuable.
I don't agree with the author's idea that an easy IP address is especially valuable. They're not making any money when technicians use their service because the IP is memorable...