Same reason Yoshida himself acknowledged that all classes were basically "the same" and nothing "unique or specialized" about them and he wanted a change.
You're right. Those space-alien-suit wearing, pole jumping, backwards leaping, long-range melee-ing dragoons aren't unique enough. They gotta have wyverns or else they're all JUST like those fist-weilding, close-ranging, dodge-happy, punch-kick-combo-ing monks.

:|

It's a DD class. Dress it up however you want, it will always be a DD class. The point of a DD class is to deal damage, wyvern or no. Adding a Wyvern pet will just only satiate the XI qq'ers who feel entitled by their 5-hour-waiting-for-party-loldrg days.