Albert Barnes Commentary 1 Kings 12:20

"And it came to pass, when all Israel heard that Jeroboam was returned, that they sent and called him unto the congregation, and made him king over all Israel: there was none that followed the house of David, but the tribe of Judah only." — 1 Kings 12:20 (ASV)

After learning what had happened at Shechem, the first act of the Israelites was to assemble the great “congregation” of the people . In this formal and solemn assembly, the crown was declared vacant, and a king was elected to replace the monarch whose authority they had rejected.

The congregation selected Jeroboam. Several factors pointed to him as the most suitable man for the position: the rank, talent, and known energy of the former exile; his natural hostility toward the house of Solomon; his descent from Ephraim; his knowledge of fortification; and the friendly relations that existed between him and the great Egyptian king. According to the Septuagint, if Shishak had not only protected him from Solomon but had also given him an Egyptian princess in marriage—the sister of his own queen—his standing would have been so high that no other Israelite could compare.

Furthermore, the more religious segment of the nation would have remembered the prophecy of Ahijah, and this would have secured their support for Jeroboam. Therefore, every motive, both political and religious, combined to recommend the son of Nebat for the votes of his countrymen.