I kind of agree, that some would probably see sense and acquiesce.
But, dont forget, for some peoples\races\species, they have other things to consider:
- History - Their family may have been rulers for donkey's years. Maybe they'd rather die trying, than not try at all.
Conversely, they may have just taken power, so do not want to give it up without a fight.
- Pride\Ego - Perhaps they think they're smarter than the conquered peoples. Perhaps they think they have better troops, greater numbers, superior training, etc.
- Culture\Not wanting to be seen as weak - Perhaps they think that, if they roll over, the new Khan will lose all respect for them and assign one of the ruler's underlings as head of state. Or, say they get control back, the underlings who see them as weak may start a rebellion\civil war. If all underlings see them as weak, it will be over very quickly.
Not to mention, if they are isolated & don't welcome contact with other fiefs, they might not actually even know that MC has conquered all others.
This also applies to any who have recently taken power & do not have the networks established to receive such news.
Many justifiable reasons to not roll over.