Despite the differences between sunni and shias I see mostly shia scholars are never against a sunni shia marriage and sunni scholars are always strictly against it. Why is that so?

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Question: Can a Shia man or woman marry a Sunni?
Answer: If there is fear that he or she might be misled, it is not permissible. The marriage is not advisable even if there is no such fear because of the consequences that the marriage may have for him/her or his/her children in the future.

The above is answer from Ayatollah Sistani.

Despite the differences between sunni and shias I see mostly shia scholars are never against a sunni shia marriage and sunni scholars are always strictly against it. Why is that so?

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Jurisprudentially there is no problem with it unless the person marrying a Shia is a Nasibi. However ethically a number of factors need to be kept in consideration and assurances taken etc
Re other Muslim scholars, they would need to be asked themselves. Shia Scholars have also by far been the most advocates of unity for over 1300 years.