Symfony was the backend framework with the most contributors in 2019[1], and Laravel is seeing some huge growth in the PHP community. I think there's a lot of hype for the modern improvements in the language, and it's quite easy to manage a stateless web server in production making it a safe technology to use.
Doesn't make it the best backend framework of 2019!
In all seriousness, I think this has more to do with the ease of PHP development comparatively to other stacks. Even Python (arguably just as easy to learn as PHP), has more overhead here.
For context for talking off the cuff, I’m not certain that this equates to a raw quality, having used Symfony professionally for a few years I found there where a lot of limitations, particularly around the dependency injection and event dispatch system, that required a lot of work to keep performant because of the inherited limitations of PHP as well as what I feel were not the best design decisions around this idea, but I’m glad so many others have had great success here
