In the MCU, Hela describes herself as, among other impressive titles, “the Goddess of Death.” The writers might have felt a second Goddess of Death would have been too similar.
This is speculation rather than something I have a quote by the creators to back up, but: I think Thanos’ original motivation simply would not have worked in these movies. Death kept her dignity in her first comic story by sitting there quietly the whole time and ignoring her abusive narcissistic stalker, no matter how powerful he was or what he did. It wouldn’t have played nearly as well on the screen. Even if the director could have made it work with great cinematography, what worked for an over-the-top comic-book supervillain created fifty years ago and a minor female character who was really less of a character than a passive object of desire, reflecting in large part the adolescent male intended audience’s fear that girls would reject them, wouldn’t be acceptable to a movie audience thirty years later. They especially could not have kept any degree of sympathy for Thanos and had him act the same way.
The plot of the movie would have made no sense with his original goal and how he pursued it, either. Thanos from the comics kept obsessively thinking of more and more ludicrous ways to try to get her to pay any attention to him. One—not even the craziest—was to murder half the population of the universe, but that was only in the hope that, finally, Death would reward him for such a gift, and when she did not, he only got angrier. He surely would never have destroyed the Gauntlet without having what he wanted.
In the movie, though, he could not have kept trying to top the last thing he did for five years straight, starting with the Blip. Even if the writers had plenty of ideas and the special effects to show them, the Blip was supposed to be the great loss that the heroes were mourning as they tried to go on with their lives. The story could not have had Thanos do a hundred other things, or even one more thing, worse than the Blip. Thanos from the comics would not even have been motivated to try to stop them from undoing the Blip. He never really cared about it and it hadn’t accomplished the one thing he did want. And Death could not have been changed without creating an entirely new character from scratch, since in the original, she is not much more than a beautiful statue, never saying or doing anything that we see.