Because they either received bad inputs that defeated failsafes, or the pilot was not properly aware that handling characteristics had changed that and doing things the old way would put the plane into a bad state.
Specifically, there were no failsafes implemented. No cross-checks were performed by the automation, because a dual sensor system would have required simulator time, which Boeing was dead set on not having regulators require in order to seal the deal. The pilots, as a consequence, were never fully briefed on the true nature of the system, as to do so would have tipped the regulators off as to the need for simulator training.
In short, there was no failsafe, and pilots didn't by definition know, because it wasn't pointed out. The "Roller Coaster" maneuver to unload the horizontal stabilizer enough to retrim was removed from training materials aeons ago, and a bloody NOTAM that basically reiterated bla bla bla... use Stabilizer runaway for uncommanded pitch down (no shit), while leaving out the fact the cockpit switches in the MAX had their functionality tweaked in order to ensure MCAS was on at all times, and using the electrical trim switches on the yoke would reset the MCAS timer for reactivation to occur 5 seconds after release, without resetting the travel of the MCAS command, resulting in an eventual positive loop to the point the damn horizontal stabilizer would tilt a full 2 degrees per activation, every 5 seconds. while leaving out any mention of said automation.
Do not get me started on the idiocy of that system here, as the Artificial Stupidity in that case was clearly of human origin, and is not necessarily relevant to the issue at hand.
