Because very probably, the man's father was damned;
Secondly God did this to protect the young man from the corruptions of unbelievers.

Allow those unbelievers to bury him. But you, follow me.

Church Fathers:
St. Augustine: As much as to say; Thy father is dead; but there are also other dead who shall bury their dead, because they are in unbelief."

St. John Chrysostom: "This moreover shows that this dead man was not his; for, I suppose, he that was dead was of the unbelieving. If you wonder at the young man, that in a matter so necessary he should have asked Jesus, and not have gone away of his own accord, wonder much more that he abode with Jesus after he was forbidden to depart; which was not from lack of affection, but that he might not interrupt a business yet more necessary.

St. Hilary of Poitiers: "Also, because we are taught in the beginning of the Lord’s prayer, first to say, Our Father, which art in heaven; and since this disciple represents the believing people; he is here reminded that he has one only Father in heaven (Matthew 23:9),[5] and that between a believing son and an unbelieving Father the filial relation does not hold good. We are also admonished that the unbelieving dead are not to be mingled with the memories of the saints, and that they are also dead who live out of God; and the dead are buried by the dead, because by the faith of God it behoves the living to cleave to the living."

St. Gregory : "The dead also bury the dead, when sinners protect sinners. They who exalt sinners with their praises, hide the dead under a pile of words."

OAM4J, Lalasticlala