Countries like Usbekistan (90% Muslim), Kazakhstan (70% Muslim), and Kyrgyzstan (80% Muslim) were never part of the Caliphate or the "Arab World" in the strictest sense, so how did they become so predominantly Muslim? A lot of autonomous republics in the Russian Federation show similar numbers. And what kept other Altaic states, such as Mongolia, from doing the same?
Actually, most of the former Soviet Central Asian republics were once part of the Umayyad and Abbasid Caliphates. See, for example, this map of the Arab Muslim conquests from the reign of Abu Bakr (d. 634 CE) to the later Umayyads (661-750). The regions that now consist of the nation-states of Central Asia were historically known by different names, some of which are still used today. Northeastern Iran, most of Turkmenistan and northwestern Afghanistan were known as Khurasan. The delta region of the Amu Darya river as it enters the Aral Sea was known as Khwarezm. Ma wara' al-nahr corresponds to Transoxania, that is, the land beyond the Amu Darya and including the historically important cities of Bukhara and Samarqand. All these aforementioned regions were under the control of the Caliphate by the time the Abbasids overthrew the Umayyads. In fact, during the last year of the Abbasid Revolution (750), Muslim armies defeated the Chinese Tang Dynasty at the Battle of Talas - Taraz (Talas) being located near the modern Kazakhstan-Kyrgyzstan border.
These regions were not marginal to the development of Islamic civilization. Muhammad al-Bukhari, the author of one of the two the most authoritative compilations of Sunni hadith was born in Bukhara (Uzbekistan). Ibn Sina (Avicenna) was also born in Bukhara. The original center of the Abbasid Revolution was Merv (Turkmenistan). Muhammad al-Khwarizmi, from one of whose book the word "algebra" comes from, was born in Khwarezm. Abu Rayhan al-Biruni, was also born in Khwarezm.
Also, it should be added that the ethnic and linguistic map of the Central Asia was quite different at the time. Most of Central Asia under the rule of the Caliphate was Iranian, that is, the people there predominately spoke Iranian languages such as Sogdian and Persian. This is why if one reads biographies of, for example, al-Bukhari, he is described as “Persian” despite being born in modern Uzbekistan - although, it should be mentioned, there is still a large Persian-speaking population in Uzbekistan. Indeed, he was descended from the landed Iranian aristocracy (dehqan) and his great grandfather converted to Islam from Zoroastrianism.^1 Turkic migrations from the north changed the ethnic and linguistic makeup of Central Asia. With the exception of Tajikistan, where the national language is essentially Persian (they call it "Tajik"), the national languages of Central Asia are Turkic.
The Islamization of Central Asia is a complicated topic. Muslims in Central Asia are almost all Sunnis who follow the Hanafi madhhab. The exception to this rule are small Shi'i communities in places such as Badakhshan. The non-Muslim population is mostly a result of Russian colonization and Soviet policy. Sunni Islam and the Hanafi madhhab were propagated in the region by the Samanids, a semi-autonomous Iranian dynasty invested as rulers on behalf of the Abbasids.^2 The Samanids were also responsible, in part, for the conversion of Turks north of their state. For this reason, when the Seljuq Turks conquered their way southwards, they were predominately Sunni Hanafis rather than other religions.
