The answer lies in the methodology of how Sharia is compiled and interpreted. Islamic law is extremely path dependent, so that theoretical inferences are drawn from what is clearly referenced from the Prophet's life and applied to new situations, which leads to new institutions, like. slave-soldier recruitment. Inferences drawn from these are applied to newer situations and so on.

This is how it went: The earliest Holy Wars were fought by people organized as clearly segmented tribes under the leadership of their chiefs. Alongside them fought believing slaves from places such as Ethiopia and Persia. There was a requirement for every able believer to fight. This much is recorded in the Sunnah.

Now, from this it can be inferred that there are 3 legitimate ways of recruitment: slaves brought along by their masters, allied tribes recruited as a whole and those who are bound as Muslims to a religious duty.

Apply this to the practical situation existing in the 9th century and you get the answer. Out of these 3 options, tribes were difficult to manage and you didn't want to empower the common folk Fellahin too much. So slave-soldiers directly owned by the Sultan became most convenient.

Yes, there are sects. The Shia-Sunni split is the result of a fracture among the very earliest companions right after his death. But within the Sunnis (who are and always were the vast majority) doctrine is pretty much the same. Only minute details of ritual vary, not enough to profoundly affect political structure. Even Iran wasn't Shia majority until very late and Egypt remained Sunni through decades of Shia rule.

Reality is messy and complicated. It is true that individual Muslims, especially in the modern world are not quite as observant of the prophetic example because of practical reasons and social change. And you'll find variations where people in the heartland of the Muslim World (Arabia, Persia, Pakistan) tend to be more observant than those in the peripheries (Indonesia, Turkestan).

However, all I implied was that the ideological framework of Sharia law greatly affected the actual political, military and social structure of the Caliphates and Sultanates. How could it not be? In practice, it's true that it was impossible to follow rulings to the dot, and rulers had leeway, but to deny that there weren't any commonalities in how kingdoms of this specific period in this specific area operated is missing the forest for the trees. The commonalities I pointed out were quite universal in the Middle East from the late Abbasid period until the 18th century. Even the far away Moroccans were using Slave-Soldiers from the Sahel. There was always at least a nominal Caliph present until WW1, so there's that. Battle strategy could be different but recruitment strategy could not, free tribesmen HAD to be recruited as whole tribes, while slave-soldiers could be better organized. Peasant conscription was rare, as elsewhere in medieval times.

I'm no professional historian. I don't know precise dates and the names of many dynasties. But you can't deny these common strands of history.

It's the example set by the initial holy wars fought by the Prophet and his companions. The tradition in Islam is to as closely imitate these and other little details of the lives of the earliest Muslims. It's stupid from a Christian perspective but it was and very much still is a thing in the Muslim world. Everything has to be referenced back to the way the Prophet did things, or it is illegitimate.

I know it's not fashionable in the West to make any generalizations at all, and this is why my analysis was being dunked on, but living in an Islamic culture, I have firsthand experience of this