Well one big difference is that PPP loans were a one time thing, they are gone now and not coming back for a very long time.
If you paid off all student loans today, there would be more tomorrow, and every day after that. If we do it once, we basically have to keep doing it until school is completely paid for and student loans are no longer a thing.
Forgiving student loans just has a much bigger and longer lasting financial obligation than one time PPP loans.