The current in the forward bias is known to be more (~mA) than the current in the reverse bias (~mA).

Consider the case of n-type semiconductor. The majority carrier (electron) density is larger than the minority hole density, i.e., n >> p. 

On illumination, the no. of both types of carriers would equally increase in number as 

n' = n + Δn, p' = p + Δp 

But Δn = Δp and n >> p 

Hence, the fractional change in majority carrier, i.e., Δn/n < < Δp/p (fractional change in minority carrier) Fractional change due to photo-effects on minority carrier-dominated reverse bias current is more easily measurable than the fractional change in majority carrier-dominated forward bias current. Hence photodiodes are used in reverse bias condition for measuring light intensity.