Credits are already used for Legendary Multiverse, Ability and Shader-granting Multiverse events. If you had to use credits to buy specific Gear set pieces then people would hate the fact that there are too many things that cost so many credits and they don't have enough for everything they want to spend them on.
Plus, from a business perspective, NRS wants people to continue playing their games as long as possible and if they can just buy specific Gear pieces with credits then they don't have to keep playing and getting boxes to get that Gear and their playtime decreases significantly. Look at the Legendary Multiverse/Gear, same reason you can't just buy that either. Wouldn't be a good decision.
Doesn't matter if it's source crystals, credits, regen tokens, whatever. Giving people direct access to the best Gear without having to grind goes against the whole point of the fairness of the Gear system.