why does this 91 Buick Regal keep blowing crank sensors

Have mechanic check reluctor @ sensor. Surely this has been checked but may mention it anyway. Are you being charged for each part?

I'm not sure what you mean by "blowing", other than they electrically short & no longer work. If so, check the harness running down to the sensor that lies against the engine timing cover & where it makes its exit out of the main engine harness for broken wiring insulation. It sounds like you have a 'short to ground' or 'short to power', basically wires touching things they shouldn't. Also inspect the connector itself for broken insulators & bent contacts. Find where its touching, and wrap it up with elec tape, & you'll prob have solved your issue. If these things are atually getting damaged on the end where the pick-up is located, you have a problem that may only be fixed by replacing your crankshaft pulley assy (it has a steel ring on the inside that may be damaged).