Leviticus 17:9
And bringeth it not unto the door of the tabernacle of the congregation, to offer it unto the LORD; even that man shall be cut off from among his people. And bringeth it not unto the door of the tabernacle of the congregation, to offer it unto the LORD; even that man shall be cut off from among his people.
God positively commanded that no sacrifice be offered except at the door of the tabernacle at Shiloh. To build another altar elsewhere for sacrifice and worship is rebellion against the Lord and departing from Him. Such an altar sets up a rival to the appointed tabernacle. It leads the people away from the true faith.
Such disobedience brings the wrath of God upon the whole nation. Therefore the tribes east of the Jordan could not keep such an altar in their own land. If they felt unable to live without an altar, they must leave their possession and be welcomed to share in the possessions and privileges of their brethren on the west rather than disobey.