It is standard design practice to inhibit indications when that indication is considered to be unreliable - and the AOA below 60 knots is not considered to be a reliable indication. That a "professional pilot" might manage to stall the aircraft so badly that airspeed dropped below 60 knots while stalled was probably not considered to be a credible scenario.
Until it happened...
Now that the designers know that there are "professional pilots" out there that will stall an aircraft that severely, the designers can take that into account with creative logic - e.g. when stall warning "true" and airspeed greater than 60 knots, if airspeed drops below 60 knots while stall warning remains true, keep the alert active.
However my personal opinion is that Bonin simply shut out the stall warning indication - literally didn't hear it - because it didn't fit what he thought was happening. If correct, something like a stick shaker would have been more effective...