Because Sarada carrying Karin's DNA doesn't mean Sasuke had physical contact with Karin or loved her once..it could be that Sakura's DNA wouldn't give Sarada sharingan that's why they used Karin (uzumaki DNA) to create Sarada in a medical lab.
Because here Kishi is trying to tell you something,the new Uchiha family is based completely on love..
Sakura loves Sasuke,that's why she's raising his child..Sasuke loves Sakura that's why he married her and gave her the Uchiha name..and they both love Sarada.
they never need to be connected with blood,they are connected with feelings,and it was for Sarada's good to carry Karin's DNA so she could get sharingan,and that's the whole purpose of this Gaiden Manga.
Because here Kishi is trying to tell you something,the new Uchiha family is based completely on love..
Sakura loves Sasuke,that's why she's raising his child..Sasuke loves Sakura that's why he married her and gave her the Uchiha name..and they both love Sarada.
they never need to be connected with blood,they are connected with feelings,and it was for Sarada's good to carry Karin's DNA so she could get sharingan,and that's the whole purpose of this Gaiden Manga.