Simba grew up and matured, simple as that. People in real life can change within the span of the year. So what makes his mannerisms and speech so surprising from transitioning from cub to adult ( putting aside his traumatic life experiences and 'Hakuna Matata' lifestyle )? Nothing. That would literally be like asking an elder raised in this generation why they no longer say "Swag, yo, dog, bro, etc". Because they realized it was stupid and quit doing it. Or like asking a pre-teen why they don't babble like a baby, drink from a bottle and play with their toes. Because people grow out of childish stuff and grow up.
You seem to be very nit-picky with these topics of yours.