How come engine stalls in low gear when going uphill?

Your engine produces torque(turning power) and revs, and these are related. You can't get more than a certain amount of torque out of the engine at a given number of revs. If you hit a slope in the wrong gear/at the wrong speed, then forcing the car forward will require more torque than the engine can deliver, and it will stall. It's very much like trying to ride a bike uphill in the wrong gear.