What is symfony, and why did it cause people to drop the platform?
Symfony is a PHP framework. https://symfony.com/
It caused much of the internal of Drupal to be re-written. This included how it was extended. With previous major versions you learned about new features and APIs. They followed mostly existing design patterns so it was easy to learn and updates your extensions for. With Symfony you had to learn whole new systems and ways of doing things. It was like learning something entirely new. And, porting extensions to it was far more work and time.
Also, the updates made Drupal slower while consuming far more system resources for the same thing. This increased costs to operate.
Wow, that's a very major change and a big expectation for Drupal admins.
Drupal is a web CMS that empowers people without extensive coding experience to do some pretty amazing things. A front end developer at a small consulting company can build a useful experience pretty easily.
And if you are a developer you can extend it extensively.
It hit its peak before some of the extensibility features hit WordPress.
When it moved to being symfony based with Drupal 8 the shift was so great for this audience that Drupal hit its peak usage [1]. It's been in decline since then. Drupal 7 is still the mostly widely used single version and BackdropCMS came long to support that crowd (a Drupal 7 fork).
I look at this as an exercise in knowing your target audiences.
[1] https://www.drupal.org/project/usage/drupal