Why is J1 dominant in Arabs?

[Opie]

If earliest Semites are Natufian which is associated with E haplogroup, why is J1 dominant in Arabic countries of Middle East.

[ShieldWolf]

Origin and diffusion of human Y chromosome haplogroup J1-M267

The theory is that they were successful goat herders from Eastern Anatolia and the Zagros mountains that migrated south into the Levant, Arabian peninsula, and North Africa.

[chinshen]

If earliest Semites are Natufian which is associated with E haplogroup, why is J1 dominant in Arabic countries of Middle East.

Because being Semitic is just a linguistic and not related to DNA.
It has been historically recorded that during certain circumstances some ethnic groups changed their language family, but of course they could not change their DNA and also due to admixture overtime would have an effect too.

[Miko]

Because the earliest Semites aren't haplogroup E. You're mistaking them with Natufians, a massive 4000-6000 year gap and different sides of the Southern Arc (one in Canaan or the southwest, the other in Central Irak and the adjacent desert in the east).

Haplogroup J1 was the one who ended the Neolithic and gave rise to Antiquity, history and civilization (and the Metal Ages).
They did so at the gates of the Arabian Peninsula. First in southern Irak (Sumer and Mesopotamia) and then expanded it to the Nile delta (Egypt).
Controlling the two entry points to Arabia, they became the Arabs.