It was a very simple document consisting of just six commonly used functions for tokens, and two events to trigger after certain functions are invoked. Because it was so simple and because developers were very interested in deploying tokens on Ethereum, it quickly became popular with developers, who started using it as a blueprint they could just copy and paste for any token. If you’ve ever tried to create a website, you’ll know how much easier it is to just take a template off the shelf instead of creating your own. ERC20 did something similar for tokens.
