The best LGBTQ character award is chosen by public vote. So the success of Baldur's Gate and popularity of Shadowheart as a romance option probably played a big part. She probably wouldn't have gotten the award if she was a lesser known character from an indie studio. There is an Authentic Representation category, awarded by a panel, which focuses more on the authentic LGBTQ experience.
In a setting like Forgotten Realms it's also more challenging to write a character arc that represents the LGBTQ experience of our own world, since homophobia, biphobia and transphobia is not institutional and structural in the way it is in our world. But the personal struggles of characters such as Shadowheart, Astarion, Lae'zel and Wyll does serve as an allegory of the journey of discovering yourself as something different than what your surroundings expected and breaking free from those expectations.
I think they did far more than simply insert playersexuality into the characters. With the stories, the notes, the voice acting, they became fully realised in an entirely different way than Housecarl Lydia.
