Why did Islam not spread among the nomads of the Great Steppe in the 8th century (during the Arab conquests), but later many of them converted to Islam?
The steppe dwellers did not want to submit to the authority of the caliphate while he was conducting the aggression. When the Caliphate weakened, many of them converted to Islam for the sake of alliance with Muslims against their non-religious neighbors and to strengthen the power of their rulers.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.