The main reason NPCs have a harder time establishing control over monster territories is due to the current world situation and the different circumstances the two sides face.
The monsters have a superior overall strength, hence they're always the attacking side. However, the NPCs have fewer territories but due to the widespread castles they've built, they're normally hard to breach. This situation has led to the NPC side being more defensive and less offensive.
Conquering an NPC territory is difficult, but if it can be achieved, it would pose no difficulty for monsters to maintain control over it.
Conversely, NPCs can easily seize monster territories in the beginning with little effort, especially those near their own castles. However, while the initial conquest might be simple, holding onto the territory is a different and much harder story altogether.
