why is the concentration of helium increased in some gas mixtures that divers breath in order to reduce the partial pressure of oxygen?

Because of oxygen toxicity.

Explanation:

Oxygen toxicity.

Oxygen has been known to cause central nervous system (CNS) toxicity when the pressure gets higher. This toxicity can cause convulsions, and this can cause the diver to drown.

The current standard maximum safe oxygen working pressure is 1.4 atm ppO2. This is equal to 56.5m/185′ when breathing air (21% O2 / 79% N). Below that depth the fraction of oxygen in a breathing gas must be reduced, and in order to maintain this maximum pressure of 1.4 ppO2, Helium must be added.