The city of Sardis was one of the great cities of the ancient world.
The city's history goes back at least a thousand years before John arrived in Asia, so John has a deep well of colorful history to draw his analogies.
The city of Sardis also happens to be mentioned in the Old Testament. John, of course, zeroes in on this reference and brings a message directly to the church.
Studying Sardis first - even though it is the fifth church on the list - provides us with fantastic imagery of how John connects the local context to the Old Testament text.