As they are a small studio, they did not have the money to buy all dnd licensed content upfront (Wizard if the Coast makes it quite expensive). When the game was a success, they added dlcs for additional races/classes they could now afford.
Lost Valley and Palace of Ice add new campaigns (they are ok but I enjoyed the base campaign more).

If having all the races and classes from official dnd content matters to you, I would recommend the Unfinished Business mod (you can find it on nexusmods). Personnally I played the first campaign with the limited options from the base game and I still had a real blast :)