I'd say PDF became a standard precisely because it was, at it's introduction, difficult to edit or otherwise modify. Even today, it's still very difficult to send a document from one person to another with any degree of reliability using anything other than PDF or an image, which isn't searchable. PDF has always been very easy to create. Fast forward a few years and now that PDF is a standard and PDF files are used frequently to exchange documents, people need to be able to work with them, extract information, search them etc. Basically, it became a standard well before the general public discovered that you need specialized tools to modify them. The other question I'd add is "difficult to edit" compared to what? An image? A Word file? A PHP driven web site? A PDF file is a single file that can package text, images, vector art, fonts, video, audio, 3D models and more. There's going to be a certain amount of complexity there, just like other formats that support such a wide range of content.
For many users, particularly those who are new to the world of document editing, they'll need tools to do basic editing. It's a standard so that people who aren't so experienced with document editing tools can get used to them and be comfortable with their operation. There's probably another, more recent reason as well; in the past, I believe PDF was viewed as a type of computer-readable code that was not open to modification. And finally, there are two reasons that lead me to believe that PDF is getting more widespread (I just made up some of them. I did not get those numbers from any sources. I might be wrong): 1. More and more people are starting to use computers and other technology. Most who do these things are very computer-literate or have had some formal training in those areas. They've begun to make use of things other people.