Why doesn't the Imam complete the Taslim in prayer by saying "wa Barakatuh" after "Peace be upon you and the mercy of Allah"?

Scholars differed regarding the استحباب (preferability) of adding "wa barakatuh" (and His blessings) to the taslim (salutation) at the end of prayer. Some considered it preferable, while others did not. Most hadiths mentioned the phrase "As-salamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah), and the Prophet's (peace and blessings be upon him) practice in most instances is the most preferable. Some jurists held that it is better to restrict oneself to "As-salamu alaykum wa rahmatullah" because its narrators are more numerous and its chains of transmission are sounder. They considered the addition of "wa barakatuh" permissible if it is not adopted as a regular habit.