- It was the only way to throw together a webapp easily for a long time.
- It is actually pretty efficient for this purpose.
- Much later Python and Node became viable alternatives.
- As soon as they did, people started shifting to them, and today few people would start a new project in PHP.
So according to my (not very informed) understanding, the history of PHP supports my claim that inherent quality (relative tot he alternatives) is more important than network effects.