They rely upon WotC because they are using a license from them in which they are going to develop the game from. As it was stated by both Mike Mearls and Swen Vincke they worked together in creating the "Baldur's Gate story". As for WotC advertising the PnP module that is supposed to take place is part of the overall reason. To get people interested in DND which Baldur's Gate is. A D&D computer game. I have a feeling that BG3 is going to play closely like a tabletop game that the transistion from the computer game to the PnP table top game will be easier for players and vice versa.
Most Game companies advertise a year or two before a game is released. It was leaked before it could actually be advertised first.
