@TNT261:
The focus on the kingdom and demon army is neccessary if you want to have some of the characters involved with those two parties make another appearance or switch from either of those places to the MC's family like the former demonlord did.
As such as someone who also reads the novel I just want to let you know that the manga is following the novel.
The novel is written in this way with changing PoVs, too.
And while it might be a bit strange these different scenes usually end up playing into one another.
For example What the demon army does - e.g. where they plan to attack - can affect the other characters if they happen to be there.
Or another example the "blonde hero" stole from the demon army so naturally they will pursue him to get those items back.
So if the "blond hero" goes somewhere close to the MC then there is a chance that the demon army pursuing him will run into the MC.