In the 1923 compulsory population exchange between Greece and Turkey, most Muslims living in Greece were forcibly moved to Turkey, and most Christians living in Turkey were forcibly moved to Greece.
The specific exceptions were Christians living in Constantinople (Istanbul), and Muslims living in Western Thrace, as well as Muslims who are part of the ethnic Albanian community.
I am searching for the reason these specific groups were exempted. I understand the exception regarding members of the Albanian community, but I am trying to figure out the reason the other two communities were exempted.
I found a book chapter called 1923–1947: Exchanging Populations and the Aftermath, which seems to indicate that the Muslims of Western Thrace were excluded from the exchange to create a symmetry in terms of the Greek Orthodox population in Istanbul. However, I do not know if it mentions the reason the Christian population in Istanbul was to be excluded in the first place. I do not have full access to the manuscript.
I was unable to find any other sources which mention the reason.