Why did it take so long for deep networks to evolve?

Since the 1960s, deep networks have existed, but training them has proven difficult. Backpropagation made its debut in the 1970s. So, deep network topologies did exist, but they were difficult to train without advancements in technologies that we use today.

Fortunately, due to the availability of computer resources, new techniques for pre-training neural networks have emerged. Imagine how far higher education can take deep learning networks.