Because now you can have two classes and considering the mixed skills in the og it seems that now each character can use and master two classes at the same time, each class has it own set of skills that characters can learn like in the OG game, but now by mixing two classes characters will have access to exclusive skills depending on the two classes being used.
EX: While using both Dancer and Priest we will learn Hustle at certain class level (1-8) but if we change the Priest to a Warrior we will learn Sword Dance but won't be able to use Hustle, while the skills learned from single classes (Sidestep from the Dancer, Heal from the Priest and Dragon Slash from the Warrior) will be permanent not matter if we change classes.
The 3DS version was a mixed bag for me but the class system was way better than the OG, but this shit ain't bad either.