Why all muffisil busses from Tiruvallur are going to Chennai via Poonamallee?

Just to help commuters in that route, I don't think all buses go in that direction, 571 from Thiruvallur to Chennai goes through Thirunindravur, Ambattur, Dasaprakash and Broadway. 572 from Thiruvallur to Ambattur is another bus. Poonamallee route is more busy compared to above route. and there are local trains to help commuters in Ambattur route.