It's all based upon what they have seen in the USNews list, which includes factors for total endowment and alumni giving rates. Since public universities have less need for both of these factors (due to the public funding they receive) they naturally score lower on these two parameters, which drags down the ranking for the publics.
Here is a ranking that does not include alumni giving and endowment. MIT is ranked #1, UCLA is #2, and Harvard is way down at #16. More emphasis is given to student achievement rather than endowment size. The USNews rankings are shown as well.
"The Washington Monthly College Guide " by The Editors