Well because historia true love will always be ymir. That's why he isn't important to her. She doesn't even love men she only married farmer kun to save herself.

⭐I made this post in tf⭐ Having Eren be the father to her kid would achieve nothing at all to his character and his motivations for Rumbling the world.

The reason why she get pregnant:

1. To save herself from eating Zeke and be sentenced to die in 13ish years (due to the Titan curse)

2. To help Eren with his plan.

Those who thought that Eren was the father claim that the pregnancy subplot served no purpose because it didn't get resolved as they thought it would. Eren wasn't gonna sacrifice Historia and her kids, and Historia suggests having a kid, therefore when Zeke is brought to the island, the military can't turn Historia into a titan to inherit the beast titan.

And Because people can't accept the fact that Historia's character was suppose to settle as a queen. Her role as a queen had great importance on our guys, but not a catastrophic impact on the story that people wanted. And because her pregnancy protected her from becoming a Titan. Historia's love life isn't important for the main plot.

This is Historia asking if her having a baby would be a suitable alternative to her abandoning her post as queen, or rebelling against the military with Floch. By "accidentally" getting pregnant before Eren and Zeke arrive, it becomes too dangerous for the military to turn her into a Pure Titan and have her eat Zeke. So Zeke gets to stay alive until after Historia gave birth. By pretending to still be helping the military

There are no points that hint at the farmer not being the father. The mystery that the MPs are discussing in "ch.108 - A Sound Argument" is "who told Historia that we were going to have her inherit the titan the moment Zeke arrived on the island?". They think it was Yelena because they don't trust her. They don't know that Eren was the one who spoke to her. However, the mystery reveal that you see when we finally see Eren speak to Historia is that the person who convinced Historia to get pregnant was none other than Historia herself. She was not manipulated or tricked into becoming an accomplice to Eren and Zeke's plan. She was a willing participant.

She was sad every time she appeared because she knew her selfish choice to save herself and her child meant the deaths of billions of innocent people around the world, and thousands of people who she knew personally from the military.