Why was it difficult to maintain a kingdom in the conquered lands?

Machiavelli describes three ways to hold states that have been accustomed to living freely under their own laws. The first is to devastate them. The second is for the conqueror to occupy them. The third is to allow the state to maintain its own laws, but to charge taxes and establish an oligarchy to keep the state friendly. The third option is advantageous because the newly imposed oligarchy will work hard to secure the authority of the conquering prince within the conquered state because it owes its existence to the prince and cannot survive without his support. Thus, as long as the goal is not to devastate the other state, it is easiest to rule it through the use of its own citizens.

Hope this helps, took me awhile to type it all. :)