Why the I-V characteristic of a photodiode in reverse bias is wholly different from the I-V characteristic of an ordinary p-n junction(in reverse bias)?

pn junction used either in FB or RB as per application eg rectifier, switch etc.

photodiode a pn junction or a PIN juction (usually with wider depletion width) used in RB mode only to detect light.

Light incident on depletion width breaks the covalent bond forming electron hole pair(inner photoelectric effect) and charge flow results in current that is proportional to the intensity(amount of photons incident) of light. There is also a dark current (very small) which is observed even in the absence of light.