What would be a reason why a government would choose a unitary instead of a federal form of government?

The country is a nation-state with a small population and a strong sense of

homogeneity.

They are seeking to effectively administer governmental services due to a

large population

It is a multinational state so another form of government would reduce a

separatist movement.

The country is geographically large so they are trying to prevent instability

by providing representation closer to home.

A unitary form of government works well when

The country is a nation-state with a small population and a strong sense of

homogeneity.

Explanation:

In bigger states, it is easier to have a federal structure so as to divide the administration and do it more effectively but in small state countries that are usually homogeneous it is much more profitable to keep things centralized to make them more effective.

This form of governance is often seen in smaller counties like Finland and Sweden where the population is manageable by a small government and there is no need for separate provinces.