Because the Bible supports the practice: Praying for others.
Is there similar scriptural support for praying to Mary, angels and passed Christians? No there isn't.
Well, you clearly believe those in Heaven are not a part of the mystical body of Christ.
Maybe you should consider the following passage from Revelation (5:8-14). Again from Tim Staples:
The Book of Revelation gives us an even better description of this communication between heaven and earth:
The twenty-four elders fell down before the Lamb, each holding a harp, and with golden bowls full of incense, which are the prayers of the saints … the elders fell down and worshipped (5:8-14).
These “elders” are offering the prayers of the faithful symbolized by incense filtering upward from the earth to heaven. And because they are seen receiving these prayers, we can reasonably conclude they were both directed to these saints in heaven and that they were initiated by the faithful living on earth. We also see this same phenomenon being performed by the angels in Revelation 8:3-4:
And another angel came and stood at the altar with a golden censer; and he was given much incense to mingle with the prayers of the saints from the hand of the angel before God.
The bottom line is this: Both the faithful on earth and our brothers and sisters in heaven (and let’s not forget our “cousins,” the angels) are all acting just as Catholics would expect. Believers on earth are initiating prayers which the saints and angels in heaven are receiving. Is this the necromancy condemned in Deuteronomy and Isaiah? Absolutely not! This is New Testament Christianity.
http://www.catholic.com/blog/tim-staples/praying-to-dead-folks