Because taking a side is more important to them than reaching as wide an audience as possible. Not making any value judgment here, but that's the situation.
They've decided X stands counter to their principles, and are risking a decrease in audience.
> We have matured to the decision to leave X for a long time. We observed the growing shares of false, troller accounts that are increasingly controlling the narrative on this platform. We have seen that X not only does not fight, but even promotes hate and disinformation posts.
"We have been maturing to decide to leave X for a long time. We’ve watched the growing stake in fake, throud major accounts that are increasingly shaking the narrative on this platform. We have seen that X not only does not fight, but even promotes hate and disinformation posts."
The translation above is really bad. It was probably supposed to be "shaping the narrative", not "shaking". In my opinion, this translation is much closer to the original:
"We had been contemplating leaving X for quite some time. We observed the growing presence of fake, troll accounts that increasingly steer the narrative on this platform. We saw that X not only fails to combat but actually promotes hateful and disinformation-driven posts."
If there are only garbage, bot, racist and conspirator replies under any post you make - then it isn't useful anymore. Not for legit readers, not for the newspaper. Also, moving might be the final straw for some readers to move - which might be considered healthy by some from a democratic standpoint.
You can tell a lot about someone from their level of hatred for Musk (and Trump). In 50 years or so, future humans will look back at this phenomenon as one of insanity of our era.