Have you ever thought why all Muhaddiseen including Imam Bukhari RA, Muslim RA and entire generations of writers of Hadees called Hazrat Ali as Alaihissalam, Hazrat Hasan Alaihissalam, Hazrat Hussain Alaihissalam, Hazrat Fatima Alaihassalam?
It is because this was the practice of Sahaba to call them so. No Sabahi narranted a Hadees pertaining to these great souls without calling them Alaihissalam or Alaihassalam.
Did Sahaba not know that Allah has described Sahabis as Razi Allahu Anhum. It means that Sahaba who were much more aware than we people now knew the special status granted to these 4 great souls and called them Alaihissalam or Alaihassalam.
This practice continued among people and Muslim scholars for centuries until arrival of Wahabis (now Salafis) who despised according them special status and tried their best to snatch the honour from the 4 great souls.
My question is that when these people ask everybody to follow practice of Sahaba in everything, why they are not following this general practice of Sahaba and Imams? Ponder over it.