And partly, because it's very hard to figure out whether a child for example was intentionally groomed into a parent's sexual partner. Or if a sibling/cousin manipulated the other through familial connection. So, there Western countries just had this idea that "wait, what if... what if we just didn't need to figure out if that's the case or not?"
Which lead to the incredible stigmatization of such relations, even though nobody bats an eye about such silly things as "power imbalance" or "questionable consent" if both partners are above a certain age and not related.... if you want to babysit your population on morals or whatever do it all the way or accept that irregular relations will happen either way...
And also denying any possibility of family members ever having "real" feeling for each other, because science said that such feelings are very likely to be different (as far as I know, no paper denied it's possibility btw).
"Science" said so because nobody supports research into the opposite. They'd call it "inhuman" and whatnot to simply shut out any possibility to disprove such "common sense knowledge". In other words: People don't like it, have been taught for generations to not like it and as such refuse to even have that belief challenged.
It's not helped that because it's a taboo you'll basically never know if any couple is related with each other or if a child is the product of incest simply because socially it's suicide to admit it.
Actually, incest being a "no-no" is a very Western thing. The reason for it is partly the fact that you are very likely to produce children with disabilities, in which case the parents basically screw over their own child.
It's actually relatively high chances of some genetic abnormalities due to inbreeding simply from the genetic combinations. I wouldn't say full on mentally challenged, but the offspring will be twice as likely as the parents to eventually develop any heredity diseases.
Thanks to Iceland being Iceland it was found out that that between cousins the children have an about 10% increased chance to inherit genetic diseases, which translates to a 0.50 percentage point increase because the baseline, as far as we know, is about 5% in the general public of Iceland.
Genetic problems don't magically appear because of "genetic combinations", if both partners have no inheritable diseases it literally doesn't matter how they're related... that's a myth that stems from people trying to rationalize that disabled children even exist (and that it never could happen to them...).