Why is it its own religion?
It seem to come from Judaism.
I'm confused because Mormonism comes from other parts of Christianity. And we still belong to the same group.
Judaism is the religion based on the Old Covenant that God had with the Jewish people. This covenant was temporary and predicated on the expectation of a New Covenant:
Jeremiah 31:31
Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah
As Paul has revealed, if the Old Covenant was sufficient a New Covenant would not be necessary:
Hebrews 8:6-10
But as it is, Christ has obtained a ministry that is as much more excellent than the old as the covenant he mediates is better, since it is enacted on better promises. For if that first covenant had been faultless, there would have been no occasion to look for a second.
For he finds fault with them when he says: "Behold, the days are coming, declares the Lord, when I will establish a new covenant with the house of Israel and with the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt. For they did not continue in my covenant, and so I showed no concern for them, declares the Lord.
For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my laws into their minds, and write them on their hearts, and I will be their God, and they shall be my people
Jesus Christ, the Jewish Messiah, is the fulfillment of this promise, and so Christianity is the religion of the New Covenant that God has made with the entire world:
Matthew 5:17
"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them.
Luke 22:20
And likewise the cup after they had eaten, saying, "This cup that is poured out for you is the new covenant in my blood.
Mormonism shares a lot of similarities with Islam, in that they both claim to be extra-biblical revelation by prophets who came after Jesus Christ, even though Jesus clearly said He was the last one who would be sent:
Matthew 21:33-39
"Listen to another parable: There was a landowner who planted a vineyard. He put a wall around it, dug a winepress in it and built a watchtower. Then he rented the vineyard to some farmers and went away on a journey. When the harvest time approached, he sent his servants to the tenants to collect his fruit.
"The tenants seized his servants; they beat one, killed another, and stoned a third. Then he sent other servants to them, more than the first time, and the tenants treated them the same way.
Last of all, he sent his son to them. 'They will respect my son,' he said.
"But when the tenants saw the son, they said to each other, 'This is the heir. Come, let's kill him and take his inheritance.' So they took him and threw him out of the vineyard and killed him.
They also both teach another gospel, and preach another Jesus. Both Joseph Smith and Muhammad received this 'revelation' from angels:
2 Corinthians 11:3-4
But I am afraid that just as Eve was deceived by the serpent's cunning, your minds may somehow be led astray from your sincere and pure devotion to Christ.
For if someone comes to you and preaches a Jesus other than the Jesus we preached, or if you receive a different spirit from the one you received, or a different gospel from the one you accepted, you put up with it easily enough.
Galatians 1:8-9
But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed
As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
Scripture tells us the source of extra-biblical revelation from angels:
2 Corinthians 11:14
And no wonder, for even Satan disguises himself as an angel of light.