An important aspect of these Roman and Sassanid sites is their origin and collapse prior to the rise of the Muhammadans.
Islam “arose” during a power vacuum, not from some great conquest of those prior powers (whom is defeated, but later). The implication is that when there is empty space (re. Russia 1917, etc.) it can be filled by anything.
The experience of early Christianity is precisely the opposite.
The Byzantines made some stupid moves, a long series of them in Armenia. Maintaining a healthy buffer state should have been a priority, but there wasn’t an IQ test requirement to become Byzantine Emperor.
https://www.worldhistory.org/article/1207/byzantine-armenian-relations/
http://www.attalus.org/armenian/char1.htm
https://en.wikipedia.org/wiki/Kingdom_of_Armenia_(antiquity)