Partly because this was necessary provision for man; and partly because such seed would not be used for man's food till it had received many alterations in the earth, whereby such pollution was taken away. See 1 Thessalonians 12:24 1 Corinthians 15:36.
Related Commentaries of Leviticus 11:37
Leviticus 11:37
37 And if any part of their carcase fall upon any sowing seed which is to be sown, it shall be clean.