2 Chronicles 21:7 Commentary - English Annotations on the Holy Bible by Matthew Poole
Because of the covenant that he had made with David; for which, in 2Ki 8:19, it is for David his servant’s sake, i.e. not for David’s merits, but for God’s free promise and covenant, as it is here explained.
Consult other comments:
2 Chronicles 21:7 - The Cambridge Bible for Schools and Colleges
2 Chronicles 21:7 - Adam Clarke's Commentary and Critical Notes on the Bible
2 Chronicles 21:7 - Companion Bible Notes, Appendices and Graphics
2 Chronicles 21:7 - Ellicott's Commentary for English Readers (Old and New Testaments)
2 Chronicles 21:7 - John Gill's Exposition of the Entire Bible
2 Chronicles 21:7 - Hawker's Poor Man's Commentary (Old and New Testaments)
2 Chronicles 21:7 - English Annotations on the Holy Bible by Matthew Poole
2 Chronicles 21:7 - John Trapp's Complete Commentary (Old and New Testaments)
English Annotations on the Holy Bible by Matthew Poole

Matthew Poole (1624–1679) wrote English Annotations on the Holy Bible, completing the chapters as far as Isaiah 58 before his death in 1679. The rest of the Annotations were completed by friends and colleagues among his Nonconformist brethren. The first printing of the completed edition was in 1685, 2 volumes folio, followed by editions in 1688, 1696 (with valuable chapter outlines added by the editors, Samuel Clark and Edward Veale), and the 4th and definitive edition in 1700, the basis of all others.