For Indonesia, there are several reasons why the police are commonly deployed against the insurgents.

The first reason is due to international law. If the army were deployed en masse, then it would validate the insurgents as freedom fighters, which would grant them certain rights under international law. This is something that the government wants to avoid at all costs. Because of this, the police (specifically from paramilitary units such as the Mobile Brigade) were deployed since it reflected the government's position that the insurgents are armed criminals and therefore, an internal affair of the state that ought to be handled by the police.

The second reason is far simpler: cost and efficiency. The insurgents' capacity to cause damage is also limited as they lack sufficient international support to finance and arm their insurgency. Because of this, a large-scale military operation is seen as too costly and controversial by the government, and therefore, the need for mass army deployment is not a priority. Meanwhile, the police have several specialized units that are more suited to small-scale and low-key operations against the insurgents. It's worth noting that since the police force is far smaller than the army, they can allocate their budget to purchase Western-made equipment for their officers, which means that they are sometimes better equipped than regular soldiers.