Why In a PN junction the majority carriers are holes on the P side and electrons on the N side. Reverse bias puts the negative supply terminal on the P material and positive on the N, pulling both kinds of carrier away from the junction. That leaves a wider carrier-free depletion region with no mobile charge to carry current, so only a tiny leakage current flows. Forward bias does the opposite: it pushes carriers toward the junction, narrowing the depletion region until conduction begins.
Watch out The idea that holes and electrons combine and turn the whole diode into an insulator is wrong because only the thin junction region is depleted; the bulk P and N material is still full of carriers and conducts fine, and the diode conducts normally as soon as the bias is reversed.
Reverse bias pulls carriers apart: wide depletion region = no current. Forward bias squeezes it shut = current flows.