That was just the way the animators chose to make the two characters.
That's all.

Kovu is younger and hadn't grown out as much

It takes years for a lion to grow a full mane, Kovu is
younger than Simba was when he returned to the pridelands,
so that is why his mane is smaller. If Disney produced
another film about Kiara and Kovu's reign, his mane would be
much larger.

Simba's mane grows down to his belly,because Mufasa's mane
did the same thing.

probably because Kovu is younger than Simba and hasn't grown
his mane full length yet