why was surha Tuhid called surha Iklas?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Scholars mentioned that the Surah beginning with (what means) {Say, "He is Allaah, [who is] One}[Quran 112:1] is called Surah Al-Ikhlaas, which in Arabic means “Sincerity”, for several reasons: it is dedicated to Allaah Alone about His attributes, as no mention is made in it of any rulings pertinent to the acts of worship, transactions, etc. It only describes the Lord. It has neither a command, nor a prohibition, nor a promise, nor a threat, nor a reference to the World or the Hereafter. Furthermore, the one who recites it observes the Oneness of Allaah with sincerity and purifies himself from Shirk (polytheism).

Moreover, reciting it is a salvation from the Punishment of Allaah The Almighty (the Arabic word for “salvation” which is “Khalaas” comes from the same root as “Ikhlaas”. In it, one acknowledges that Allaah The Almighty Is free from all defects, partners and sons (the Arabic word for “free from” which is “Khaalis” also comes from the same root as “Ikhlaas”).

For more benefit, see Fatwa 88161.

Allaah Knows best.