That´s a one million dollars question. Every year predators and cold wheather kill millions of them. That´s their Nature. Think of this: one female lays 450 eggs as average. In order to keep the level of total population only one survives until complete their cicle: 449 die in all the diferent stages of their development: egg, larva, pupa and adult. And in this late stage, only stonger survive in the cold of our mountains. Finding millions of them died in our woods yearly, is natural. And "per se“ something we, humans, are not use to. Therefore, our astonishment.
This doesn´t mean they have an ensured existence: humans threat them in many ways: world pollution, logging, etc.
In the other hand, when you follow the statistics of scientists you´ll notice that every year the population numbers change sometimes dramatically, and then in the next years they increase. That´s something not easy to determine the causes of it, but Science is on its way. So far, one thing is for sure: significant variation of numbers year after year. Please, read again point D.