Why did the Internet grow at such a fast pace?

Their are many reasons why the internet grew at such a fast pace. Here are four:

Interoperability. It is designed to work with many different types of devices and systems not just connecting identical system to identical systems.

Open protocols. TCP/IP is a great example of this. When Bob Kahn worked on TCP/IP back in the 70's it was with the idea of connecting military computing resources. So he reasoned that it would need to eventually accommodate on board systems in aircraft, large mainframes, client systems, even mobile systems - not just fixed circuit systems.

In the 80's networking services were opened to commercial companies to use - previously they were government controlled.

Essential internet hardware became cheap and accessible in the mid to late 80's. Equipment like routers and modems.

As I said there are many other reasons but essentially these are what made it possible.