Ape species began to dwindle during the Miocene, leaving only a few remaining: a situation that exists up to the present. It is thought that the number of ape species reduced at that time because

The change in weather pattern caused forest to become sparse and hot weather caused the reduction of apes as they had less food and difficulty in surviving the climate.

Explanation:

In Miocene epoch volcanic mountain chains were created in Central Africa which abruptly changed the local weather.

The sea-level got drop due to global cooling of that time.

This resulted in continental drift making land connection of Africa with Eurasia.

The primates started migrating through this route.

The African forest become dry causing selective pressures in primates.

The cooler climate in Eurasia caused extinction and migration of primates to Africa in late Miocene epoch.

The hot and humid climate and less woodland caused a reduction in number of apes which were replaced by modern world monkeys.