Well... I can think of one or two reasons why Primrose would want to leave Efrafa with Hazel, than to stay there with Campion.
The first one would be the fact, that she was brought to Efrafa against her will. As I recall she was kidnapped from Redstone Warren (more likely from its surrounding area, since Woundwort would probably attack this warren, if he knew its exact location) by one of Vervain's raiding parties or something like that. Thoughts of home was the main thing that kept Primrose going while she was in Efrafa and after she regained her freedom she almost immediately traveled back to Redstone. Even though she was quite content living on Watership Down.
The second reason: Primrose was practically a prisoner in Efrafa (every non-Owsla rabbit was). She wasn't allowed to leave the warren and if she tried to excape she would probably be killed after being caught. Almost every element of her life was controled by Owsla. She could only feed at assigned time and spot. When not on silflay she had to stay in her burrow, which must had limited any occasions to socialize with other rabbits (except those, who would sleep in the same burrow). I don't think many would want to stay in Efrafa when there was an alternative.
Campion was a decent rabbit, but nevertheless he was part of the system that was imprisoning Primrose and making her life miserable. Even if she chose Campion, even if the mate of Owsla officer could get a slightly better life in Efrafa, she would still be prisoner. Just a prisoner in bigger cage.
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Same story for Dandelion. He goes out for a girl and Hawkbit comes in like some butthole and takes her away, while it doesn't seem that Hawkbit is too ready for a relationship. In my mind it's like he only wants Clover because she's the only socket available to plug himself into the night before he thinks he may very well die.
Didn't they fought over a Heather, a doe from Efrafa? I don't think Dandelion ever showed any interest in Clover and Heather left Watership Down with Moss.
As for Hawkbit's motivation for wanting Clover... This subplot didn't get enough time (it's the end of the series, so I guess it couldn't be helped) to show us, if Hawkbit was courting Clover just to have sex with her or if he really wanted to have a serious relationship. However, I'm willing to believe it was the latter.

TL;DR: It shoulda been Dandelion and Clover.
You can always write a fanfic, where they end up together.
