I think only mc's kingdom is the stongest among other celestial race after he change his kingdom's race to dark attributes. Since the celestial race originally one the weakest race, they will always have that prejudice that the celestial race IS weak.
They probably just doesn't want to accept one of the weak celestial race kingdom is strong enough to be in first place. But who know what's going on in their head? They just want bully the weak.

i have a question, why is everyone still doubting the celestials even though they are the ones at the highest floor. twice the level of the second place btw (i think…) aren’t the celestials essentially the strongest?