Why is a semi-conductor used in a Hall Effect Sensor?

I can't say for sure, but (1) in a semiconductor, both electrons and holes exhibit the Hall Effect, (2) in a conductor, the Hall Effect is harder to detect amidst the much stronger effect of traditional conduction, and (3) it's easier for a quantum effect to have a measurable manifestation in a semiconductor than in a conductor. That should let you measure much weaker magnetic fields.