The truth - terrorism made this situation happen. For years pilots have lost it, had heart attacks, nervous breakdowns etc. but there was always another pilot ready to take over. Now because of terrorism, if one pilot goes nuts and has 5+ minutes to bring the plane down they can, because to stop terrorists a pilot can lock the door to the cockpit for 5 minutes.
This is why we do not know how many other incidents with other nuts pilots have happened because the flight crews have dealt with them successfully. The industry does not want to say how regular this is, because simply it would undermine a very safe way of travelling, which is stunningly successful. This accident therefore results from the problem of restricting access to a highly dangerous area for good reason, but enabling another problem to exploit it.
I remember the story of a pilot who let their daughter into the pilots seat, and was not supervised while she pressed the buttons. She caused the auto pilot to switch off, and plunged the plane into a massive dive. The pilot fought to get back to the seat and pull the plane out of the dive but too late, and all were killed.
Jumping to the terrorist conspiracy theory all the time unfortunately is just paranoia and dishonours the good things you stand for.