rbrtrbrt
The SQLite site phrases it more precisely: "Most Widely Deployed and Used Database Engine" https://www.sqlite.org/mostdeployed.html
My first guess would be a combination of (1) making simple things easy -- by the time you start experiencing the warts and flaws, you've already been successful, and willing to work around them. And (2) (later on) ubiquitous availability on web hosters, giving it a much lower barrier to entry than other languages in the web space.
Other aspects could be a community that's welcoming to novices and autodidacts, or its integration into a "default" infrastructure (Apache, MySQL).
But it would be interesting if some aspects of the languages design itself could be shown to have contributed. Perhaps the mixing of HTML and imperative code? Not having (at first) object orientation? Could its unorthodox semantics for passing by value/reference have been helpful to beginners?
Getting a handle on such design issues for programming languages could be helpful in trying to come up with "better" languages that could be accessible to beginners and end-user programmers, and PHP is an interesting case here.
http://www.ibiblio.org/apollo/listings/Luminary099/PINBALL_G...
It's in the code repository for multiple Apollo's