There’s not really a story to this - just a student who most likely didn’t plan properly as alluded above. This class did fill up on the second day of senior registration and it’s possible the student’s registration date and time was after that. And there is some bad luck involved in that this class was usually 3x larger. However in my opinion the onus is on the student to plan this via berkeleytime.com in advance knowing that certain classes can fill up fast and to prioritize those classes earlier on. Yes it is a pain that classes fill up, but it can happen at all schools, big or small, public or private.
People get spoiled taking CS classes at Berkeley since many of the popular classes often expand to the point of taking everyone in, and I have a feeling that sometimes some students get too used to this.