I could be wrong about this, but I believe the main reason why we haven't seen many snow delays in the past ten years is that the budgets of schools are too tight to allow them. The state of Michigan is pretty strict about the number of snow days a school can take; it has them counted down to the hour. Every snow delay takes two of those hours away. As the number of snow days allowed can easily be used, and the state no longer allows those days to be made up by adding time to the school day, school administrators are hesitant to take a snow delay which costs them a full day of funding for teachers, cafeteria works, hall monitors, paraprofessionals, secretaries, custodians, bus drivers, etc., especially if they might have to pay for another full day that gets added on to the school year.
Don't get me wrong; I personally prefer that schools take the entire day rather than just have a delay. Delays cause all sorts of headaches for teachers, parents, and students alike. The problem is that administrators aren't taking full days on days when there should be delays due to similar financial issues. Instead they are put in the tough position of risking that no serious accidents will occur during those morning bus runs.
So far, they've been lucky.