Why is not every airplane equipped with 3 angle-of-attack sensors?
Triple redundancy is necessary to detect a fault and exclude it. The system then continues to operate through the fault. Double redundancy is used to detect a fault but cannot exclude it, so the system stops operating. The important fact is that the faults they actually detect are identical.

Stall events are rare and are normally not expected in flight. There is no immediate hazard if handling augmentation or stall warnings are disabled. Therefore, there is no need for triple redundancy.

Simply put, if the system detects an AoA discrepancy, it can simply trip off and stay off until it is repaired on the ground.

If the double redundant system is ideally designed, then only a simultaneous fault will escape detection. Note too that if the same simultaneous fault occurs to two sensors in a triple redundant system, then it will also escape detection because it will outvote the correctly operating sensor. Therefore, both systems share the exact same failure mode.

Double and triple simultaneous faults can and do occur with common causes including environmental factors (AF 447), maintenance errors (XL 888), and birdstrikes (US 1549). It also allows faults in the voting logic (QF 72). Both recent AF and XL fatal accidents are signs of an overreliance of buying 3 of the same box and then calling it "safe".