In Revelation chapters 2 & 3, there are 7 churches mentioned. Is there any reason why these particular churches were included in the list? Why only seven? Are they the only churches that was persecuted by the emperor that time? Why were the church in Rome or Jerusalem or other churches that we commonly know in New Testament letters are not included?
To answer this requires speculation. Such speculation falls into the “for what it is worth” category. God does not tell us why he chose these particular seven churches, so we are left to make an educated guess. Probably there is no great doctrinal or theological implication to the choice of the churches. I am sure the use of the number seven is not just an accident, as this number had such significance to the Jews–being the number of perfection. God chose seven because of the spiritual significance of the number seven. The seven churches correspond to the seven stars and the seven gold lampstands in Revelation 1:19-20. In Jewish symbolism, seven represents completeness or perfection.
I assume he chose these particular seven churches because they were all in the Roman province of Asia and because John was in Asia (actually he was on Patmos, a small island off the coast of the mainland of Asia). These seven churches represented the seven churches immediately in the area where the apostle John himself ministered. We do not know a lot about the persecutions under Domitian. What we do know comes principally from the church historian Eusebius. Eusebius describes the persecution as general, but there is not a lot of outside evidence for a massive empire-wide persecution of the church under Domitian. We can safely assume that these were not the only churches persecuted by the attacks of Domitian, but scholars cannot give a detailed account of the Domitian persecution.
In summary, seven churches were chosen for the spiritual symbolism of the number seven and the churches chosen for mention are the ones in the immediate vicinity of Ephesus where John ministered to the churches. This is why more significant churches such as those in Antioch or Jerusalem or Rome were not mentioned in the vison.