Because this is the last chapter of the Cauvery, and very few people ever see it.
The river runs 800 kilometres from Karnataka, and everything about the Tamil delta — the rice, the Chola temples, the wealth that built Thanjavur — derives from what it deposits. Most visitors see the temples and never see the river system that made them possible.
Muthupet holds the largest mangrove forest in Tamil Nadu, and mangrove is one of the more interesting habitats you can be inside rather than look at. The channels are narrow enough that the forest closes around the boat.
Add Point Calimere — a Ramsar wetland with blackbuck and, in season, thousands of flamingos — and you have two protected ecosystems within an hour of each other, on a coast almost nobody visits.
It is quiet, it is cheap, and on most days you will have the channels to yourself.