Blessings On Progeny Of Ibrahim عليه السلام
Actually saying peace on Prophet Abraham (sws) and His progeny covers it all but the reason I think that Prophet Muhammad (sws) and His Progeny are mentioned separately has some literary and historical significance. Prophet Abraham (sws) hold a special status of Imamat while Prophet Muhammad (sws) hold the special status of culminating the Abrahamic tradition to its final form till the day of judgement. Therefore, mentioning of both the start and the end separately covers the whole of the second phase of prophethood beautifully.