Isn't it also because of a change in testing methods? It seems to me that multiple choice tests are more and more widespread. These can be gamed more easily, since you can often eliminate some of the choices based on knowledge unrelated to the correct answer.

For comparison, during my own education, a couple decades ago, I don't recall having a multiple choice test ever. Maybe 1 to 4 grade in primary school. Maybe. Everything was problems, proofs, or essays.