Prior to those events, the standard protocol was to assume a diversion, a hostage negotiation and a standoff, with likelihood resolution without bloodshed. Hijacking was either for extortion or for a ride somewhere else. They would get the plane on the ground and start negotiating.
Post-9/11, the assumption is now the entire planeload is already dead and the hijackers DGAF if they get out alive. As a potential hijacker, this removes your primary bargaining chip.
Plus, the locked cockpit doors mean you can't get to the pilots. Even if you can somehow convince the pilots to get a message out, you'll get nothing. They'll just get to an appropriate airport whether or not you start killing passengers every 5min. Then, you'll just be shot by the SWAT team on landing. Moreover, in many countries including the US, the protocol now includes shooting down hijacked commercial airliners if the plane is deemed a threat to strategic targets [0].
So, since then, the likelihood of any potential reward from hijacking has gone to near-zero, and the risks have become essentially infinite.
And of course on top of that, despite the publicized failures, the security theater still substantially increases the risk of getting caught even trying to board a plane to hijack it.
