According to Civic Senior Product Planner, Rob Keough, the reason was to reduce drone when going on the highway.
He was quoted during an interview with Road&Track that "traditionally with these big flow exhaust systems, when you get up into highway speeds, you can get a lot of droning, booming, buzzing - not very comfortable for high speed cruising."
If you look closely at the car, you would notice that the two outer ones are larger than the inside one and come with straight flow mufflers, whereas the center pipe is a resonator tasked with performing a specific aural trick.
Keough explained that at low speeds, the resonator assist in generating a louder and more aggressive growl both inside and outside the car. Once you hit higher speeds, the exhaust flowing through the center pipe hits a resonant frequency and since the resonator gets filled with air, the resulting back pressure diverts the exhaust to flow exclusively through the larger outboard mufflers, thus reducing cabin noise at highway speeds.
That said, he added on that "it's not like you have zero exhaust sound, but you're not generating this extra sporty sound that comes from the resonator."