The correct answer is - Rocks that are old are hard to find.
In order for the scientists to be able to study the beginnings of the plate tectonics they need a physical proof. The problem is that the oldest pieces of rocks have been subducted sometimes in the past, thus recycled in the mantle, so the proof is long gone. The oldest rocks on the planet, usually found deep into the mainland of the bigger land masses, are very rare, and very small, and on top of that, even though they are the oldest pieces of rocks, they are not the first ones that appeared, at least that is what is thought.