Over time the per-student funding from states has gone down significantly. But that doesn't mean the state slashed funding for higher education! For University of Washington the budget from the state is almost entirely flat. Up about 3%.
Student Size: +100% State Tax Revenue: +40% State Higher Education Funding: +15%
What would you consider the "dominant factor" here? It's several factors. But I would not consider, and I quote, "a steady decrease in support for higher education on the part of state legislatures" to be the dominant factor. In fact, that statement is flat out wrong. The state legislature hasn't decreased support at all. They're providing more support than they were! The state, and the state's coffers, have no way to keep up with the growing number of students.
http://f2.washington.edu/fm/financial-report-archive
(I did a little digging last night specific to the University of Washington. Numbers from memory and a little fuzzy. But within the ballpark.)