You do realize that Triss is an established character (in the games) and Yennefer is new, right? We had to focus on her a lot more to introduce her to players who have no clue about who she is and what Geralt and Yennefer's story is. Fact still remains that she is a huge influence in his life and people needed to know why.
Additionally, throughout the majority of the game the choice is between Triss and Yennefer for the player (or both or neither). So we cannot really make assumptions before the endgame. We can however showcase the deep history between Yennefer and Geralt because it is written and accepted in the source material. Again, something that a lot of people would not understand if we didn't show and tell it to them.
But fear not, undoubtedly there will be mods eventually that further develop the romances. I am looking forward to those mods.
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There is more interaction with Triss through Part 3, especially in Novigrad.
As for the additional Yen content...refer to my post just above.