Tithing before the law

Tithing1 existed before the Law,2 in the law,3 is mentioned in the New Testament,4 and neither Jesus nor the New Testament writers ever negate it.

Levirate marriage5 existed before the Law,6 in the law,7 is mentioned in the New Testament,8 and neither Jesus nor the New Testament writers ever negate it.

Why do preachers and teachers use this argument to support continuation of the tithe, but not for levirate marriage?

[1] Tithing is giving a tenth of one’s increase.

[2] Genesis 14:18-20

[3] Numbers 18:20-24

[4] Matthew 23:23

[5] Levirate marriage is a type of marriage in which the brother of a deceased man is obliged to marry his brother's widow, when the deceased brother has no children.

[6] Genesis 38:1-30

[7] Deuteronomy 25:5-10

[8] Matthew 22:23-33