Upgrading the communicator from a handheld device to a small pin on the chest was actually a logical upgrade storywise. Scenario: You're captured by an alien race during a first contact mission. They confiscate the phaser because it looks like a weapon. They confiscate the tricorder because it's obviously some sort of electrical device. They don't recognize the communicator pin as anything but a decoration.
Consolidate all those into a single functional item. The aliens confiscate it and you've got nothing.