Or they don't want to spend half their time managing that Jenkins ecosystem when some bash scripts and literally any other CI solution out there gives you very similar benefits for fraction of the effort.
My previous company also turned away from Jenkins to GitHub actions. I'm not an infra guy and the only complaint about the Jenkins pipeline is its slowness as it copies everything from the feature branch into the deployed branch, while only a few files were changed. But I don't think it's Jenkins' fault though.