Because many of them had been with the company a long time (and were thus old), they were replaced by mostly younger people who had less experience with what makes Apple Apple.
In my personal opinion, the soul went out of the company. It wasn’t immediately when Jobs died - to his credit it took more than a decade for the momentum of his vision to die.
Today, Apple is just another multinational, nickel and diming their customer base for services revenue, cooperating with the totalitarian surveillance state (not that they have much choice in the police states of the PRC and USA in which they operate/can’t fight city hall), shipping incrementially improved products (and incrementally worse software), and misleading their customers with clever marketing. It’s just business. It used to be something else entirely.