1 Chronicles says Reuben was the firstborn, but because he defiled his father’s bed, his birthright was given to the sons of Joseph (1 Chronicles 5:1). The text also notes that Judah prevailed above his brothers and a ruler came from him, though the birthright belonged to Joseph (1 Chronicles 5:2).
In 1 Chronicles 5:1–2, the writer explains why family ranking and inheritance lines do not follow simple birth order. Reuben is named Israel’s firstborn, but he loses the birthright because he violated his father’s bed. As a result, the birthright is assigned to Joseph’s sons. These verses function as a sidebar before the listing of Reuben’s descendants later in the chapter. The writer pauses to clarify a potential confusion: why Reuben is treated as firstborn in name but not in the privileges attached to the firstborn position.
These verses explain why Israel’s family lines in Chronicles do not always follow simple birth order. Reuben is still named as Israel’s firstborn, but he loses the birthright because of a serious violation involving his father’s bed. The writer then states the result: the birthright is reassigned to Joseph’s sons.
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with 1 Chronicles 5:1, then read the surrounding section so the answer stays connected to the passage's flow.
1The sons of Reuben the firstborn of Israel (for he was the firstborn; but, because he defiled his father`s couch, his birthright was given to the sons of Joseph the son of Israel; and the genealogy is not to be reckoned after the birthright.