Why?
“15 For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, “Abba, Father.” 16 The Spirit himself testifies with our spirit that we are God’s children.”
If you can call God "Daddy", why do you have to ask Mary to petition Jesus on your behalf?
"St. Paul told the early Church to pray to those who had died and were in heaven for help."
Where?
Not only do those in heaven pray with us, they also pray for us. In the book of Revelation, John sees that “the twenty-four elders [the leaders of the people of God in heaven] fell down before the Lamb, each holding a harp, and with golden bowls full of incense, which are the prayers of the saints” (Rev. 5:8). Thus the saints in heaven offer to God the prayers of the saints on earth.
Angels do the same thing: “[An] angel came and stood at the altar [in heaven] with a golden censer; and he was given much incense to mingle with the prayers of all the saints upon the golden altar before the throne; and the smoke of the incense rose with the prayers of the saints from the hand of the angel before God” (Rev. 8:3–4).
Jesus himself warned us not to offend small children, because their guardian angels have guaranteed intercessory access to the Father: “See that you do not despise one of these little ones; for I tell you that in heaven their angels always see the face of my Father who is in heaven” (Matt. 18:10).
Because he is the only God-man and the Mediator of the New Covenant, Jesus is the only mediator between man and God (1 Tim. 2:5), but this in no way means we cannot or should not ask our fellow Christians to pray with us and for us (1 Tim. 2:1–4). In particular, we should ask the intercession of those Christians in heaven, who have already had their sanctification completed, for “[t]he prayer of a righteous man has great power in its effects” (Jas. 5:16).
As these passages show, the early Church Fathers not only clearly recognized the biblical teaching that those in heaven can and do intercede for us, but they also applied this teaching in their own daily prayer life.