fetish for pregnancy isn't the same as fetish for impregnation. I honestly don't understand the mind of a cuckquean, I'm a man, but I honestly doubt most of them have actual fetish for cuck pregnancy, because pregnancy itself isn't particuarly a sexual or emotional thing. You can fetishize over your boyfriend having sex with another girl, you can fetishize over your boyfriend having feelings for another girl, but watching another girl pregnant of your boyfriend's child doing her daily chores isn't exciting, there's not much to imagine about it, specially if you're talking about real cuckquean (the fantasy world always allows for more).
It's probably for the same reason as cuckolds do, and the majority of cuckolds have fetish for impregnation, not pregnancy. It has nothing to do with genetics nor reproduction, it's most likely because impregnation is the ultimate act of cucking fetish, it's beyond sex, beyond feelings and relationship, it's biological: you're thinking in their sex not as her being pleasured by another man anymore, nor as her having her pussy being used by other men, you're thinking on her having sex with another man in its biological level. The idea of impregnation is hot because it means your wife's/gf's reproductive system is being used in its full functionality by another man, it has another man's cum in there instead of yours after they're finished and she sleeps by your side.
I'm sure cuckqueans probably think similarly.