Unlike the book of Kings, Chronicles pays scant attention to the Northern Kingdom. Only here in chapter five, and then again briefly in chapter seven will you find them mentioned. The particular tribes mentioned in chapter five are called the “trans-jordan” tribes: those that made their home east of the Jordan river and did not enter Canaan to inherit the land.
The reader is to be surprised that this genealogy does not begin with the eldest child of Jacob, so the writer begins the chapter by telling why Reuben was not mentioned first.
But the question is: why mention these tribes at all? After all, the genealogy given for them is far from complete. Note that we are given the sons of Reuben. We would expect to follow the sons of Reuben’s sons. Instead, the writer mentions the line of Joel who is undoubtedly of the line of Reuben, but we don’t know where he fits. The same sort of disconnect occurs with the descendants of Gad. We get even less genealogical information regarding the half-tribe of Manasseh.
In a section given to genealogy, with such sparse and incomplete information, again, why mention them at all?
“Spirituality” is one of the sub-themes of Chronicles. We are told about Reuben and Gad because they found themselves in difficult straits and prayed to the Lord. God delivered them because of their faith. Though the people of Manasseh were numerous and successful in a worldly way, they were unfaithful to the Lord. Mention of these tribes, in a book that virtually ignores the northern kingdom, is a surprise and emphasizes the importance of trusting God, and prayer offered in faith.