The exclusivity choice certainly played a part in diminishing the interest in her character, but it's not the main reason at all because 35% of total players actually joined Minthara's side of the story during the opening week, a period when nobody knew anything about the game.

[Linked Image from gamespace.com]

Afterwards over time it slowly became obvious to everyone that her entire side of the story is terribly underdeveloped and buggy since Larian was prioritizing the good path, which gave that entire side of the story the reputation that still stands today. That reputation drastically worsened when it turned out she is so terribly underdeveloped with more than 1500 voice lines missing due to "a bug" according to Larian, when in reality it was them downplaying her rushed development and countless of bugs. Her dialogues broken, her romance nonexistent, her reactivity not even conceptualized yet, her side of the story not offering even 1% of new content. And it still keeps being an issue till this day.

So naturally who wants to waste their time on a clearly underdeveloped side of the story that offers nothing in return with a companion that is broken beyond belief, when Shadowheart, Lae'zel and Karlach exist and provide a much more fulfilling experience.

Simply go back to see how popular Minthara was during Early Access here and especially on Reddit when people religiously discussed every miniscule change about her, from her appearance to her lore, especially her hairstyle changes and sex scene angles. Her fandom was as religious back then as Astarion's fans today. How many threads were born since then to literally repeat the same exact complaints word for word about her entire side of the story being so underdeveloped from Early Access up until this very day. More than 3 years of feedback and what has Larian done to address it? She doesn't even have a single voice line for casting cantrips and spells.