The list mixes acceptance and graduation rates along with student survey responses about workload to get the rankings. Including acceptance and graduation rates naturally biases the list toward more selective schools.
If it were only about student survey responses about workload, the list might be a lot different. Indeed, it would not be surprising if less selective schools with high percentages of students in high-workload majors moved to the top of the list. Perhaps South Dakota School of Mines and Technology and Belmont University?
Of course, schools with additional curricular requirements, such as the military service academies as mentioned above with required military activities and sports, could also move up in the list.