Asalamu aleykum wwb
why qiraa`a Hafs from 'Aasim is the most common.
jazakalahul kheyr

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 is His slave and Messenger.

The fact that the recitation of Hafs from ‘Aasim is the most common recitation is no doubt by the predestination of Allaah. However, this does not mean that this popularity has no reasons; rather, there are reasons for it, among which is that it is an easy recitation, simple to utter, and naturally, a person desires simplicity. On the contrary, other recitations have some Imaalah (in which the Madd [prolongation] of the long vowel A is pronounced between the long vowel A and the long vowel E) and Ibdaal of Hamzah (in which the letter Hamzah is replaced with the corresponding long vowel) and so forth. It might be that the Ottoman Empire has a role in its circulation.

For more benefit, please refer to Fataawa 91213 and 91555. 

Allaah Knows best.