Other languages caught up with PHP.
As I remember it all the early languages that let you write code inline with the HTML were proprietary such as server-side Javascript built into Netscape's web server, ColdFusion, ASP, etc.
PHP was the first of these that was open source and basically competent which made it my #1 choice for making web applications in 2001. Compared to many other languages (say cgi-bin) it was pretty fast without a build step and had enough resource management that hosting firms could offer cheap PHP hosting plans which made world-changing open source products like Wordpress deployable.
It got long in the tooth quickly because people realized that to make quality web applications you had to have a part of your framework (a "router") which could serve different pages based on the inputs. For example, if you are doing server-side error handling on a form you either display a form that says "successful" or you re-display the form with error messages. You certainly can write PHP that does that if you have some discipline, but once you introduce a router you might as well write your "views" with some kind of templating system and after ruby-on-rails every language got frameworks, typically Sinatra-like, which were about as comfortable as PHP and that pushed you into having some discipline.
Its HTML-inline nature is the entire reason I ever used it. At some point in the early 2000s I came across PHP’s include, which felt like the piece of HTML that’d always been missing. Suddenly I no longer had to manually keep all that header/nav/footer code in sync across N pages! Magical.
It wasn’t until Rails rolled around that I built anything resembling a “real” web app, though. The structure and convention it brought made it much more approachable than PHP, because all the examples of more involved PHP at that point were chaos-spaghetti that was a challenge to tease apart and use as an example for learning.