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.