As this discussion came up again on the Steam forum but discussing anything on Steam is a gamble...

Why do you think Larian has not implemented the revised dragonborn from Fizbans and instead used the underpowered base dragonborn?

Not necessarily the gem dragonborn (why they haven't added psionic in a game about mind flayer is a separate issue), but the buffs to metallic and chromatic dragonborn like secondary breath weapons, more breath attacks or temporary elemental immunity.