Confused with photodiode amplifier circuit
Each semiconductor diode is susceptible to light, that's why most of the non-photodiodes are packaged in dark opaque plastic cases.

A photodiode in contrary has its big junction area exposed to light and is packaged in a transparent casing.

In forward direction, the current through a diode is so high an additional small current caused by light does not matter. But in reverse direction, the diode only allows a very small reverse-blocking current. The additional photocurrent easily can be 10,000 times higher than the reverse current in darkness.