Machiavelli's theory which rougly divided states into 2 types offers a reason.

The 2 types are

- One where a Ruler depends on his lords (decentralized) which is "easy to enter, difficult to hold"

- One where everyone is a servant of the Ruler (centralized) which is "difficult to enter, easy to hold"

The 2 contempory examples he used where France and the Ottoman Empire. Although the centralized state can wield its power more efficiently. These giant, often multicultural, empires always had a city as a seat of power, from which the empire was governed, once you topple it's "head" (remove the ruler, capture the capital) you can use the empire's existing structure to consolidate your power over your domain.

His prime example, from the antiquity would be Alexander and his conquest of the Persian Empire. Other examaples are the Seljuks in Anatolia, the mongol invasions in Asia or the Spanish conquest of the Inca empire.

The decentralized states on the other hand are diffrent. Their fractious nature makes them more easy to conquer but the state itself it held together by a lot of personal bonds (not only the lords to their rulers, but the diffrent domains to their lords) It's a lot harder to take over this system with one of your own, especially if your religion/culture is diffrent. In additon in these type of states, although they might have a capital, their seats of power, and this plural since each lord had his own, often isn't a city but a castle.

A decent example of this is the 100 year war. Although the French suffered staggering defeats in the field the weren't permanently conquered by the English. And if we stock to English history: The Scottish, although conquered, would regain their independence after a 100 years, the conquest of Ireland, if you start counting from the Norman invasion took nearly 700 years.

Although their is a lot more to it in reality but this political frame offers some explanation to your question. Offcourse every state is unique in his own way but his theory has some merit in most of the relevant cases. But as with most political frames, once you start using it it's easy to find proof to validate it's claims.

Source: Machiavelli, Il Principe.