I stopped reading the novel after few chapters but so far what I read, Vijaygarh is a small kingdom but not weak. Naugrah and Vijaygarh are rivals in the novel, the King of Vijaygarh (CK's dad) is an idiot he is manipulated by krur all the time and he does not care about his daughter at all. He wants to kill Virendra.
But even when these kingdoms are rivals, they do not go on war because they are smart people. The Kings play politics in the novel such as "lets kill the crown prince or the king". Killing the crown prince/King would be automatic failure of the kingdom even if they have the largest or the strongest army. If there is no one to lead, that army is bound to fail.
Now Shivdutt is as wicked in the novel as he is shown in the TV show. Once he hears of CK's beauty he wants her but he too plays politics rather than going on war directly.
If they decided to follow the novel A to Z, I would have been so bored with all the nonsense politics. I am happy with the changes they made.
To answer your question, in this show instead of showing CK's dad as a jerk they decided to show a weak kingdom. Else, her house would be full of negative (atleast in the show her dad loves her no matter how stupid he is)