Probably not canon but.
Horde Prime was originally operating via Hordak’s memories. Whatever Hordak saw or believed, so did Prime. He knew names and faces of princesses, histories of towns in the war, certain battles.
And Hordak thought Entrapta was dead.
So Prime did too.
Prime obviously eventually found out. But at the beginning, he assumed this purple haired annoyance was gone. He was looking for the other princesses, but not her.
Hordak, even in his last act as himself, was able to save her. Even if just for a month, a week, a day. He bought her time.
Even when he had nothing, he found a way to protect her from the evil he was facing.
Just a thought :)
Hey I still never use tumblr and for the second time I scrolled my dash when I was anxious. And for a second time you were on there with good takes. I'm at a club people watching this time, not that anyone cares