The answer is belief in Islam.
Arabs invaded the Sindh region in northern India during the 8th century. It was the Arab Umayyad Caliphate (an Islamic empire) which attempted to expand its frontiers to the East and conquer India and, therefore, started a series of campaigns against the Indian kingdoms in the East of the Indus river, but finally failed to conquer this region.
The Turkish were a nomad people from central Asia who converted to Islam in the 10th century. After that, under the leadership of Tugrul Beg of the Seljuk dynasty, they started to expand to the South-West and invaded the Khurasan region in Eastern Persia. They established the Seljuk Empire which expanded to the West into Anatolia, what today is modern Turkey.