In a nutshell, Because the C-D road (Calais to Duisburg / Dusseldorf) is the most popular route and you are guaranteed to see a lot of other TMP drivers. It's also not a bad route to drive. I often see my fellow VTC members on this route, Not so much if i venture out into Europe, Which i do on occasion.
My advice to you is that if you do not like the road when it is so busy, Just avoid peak times. Anything over 1500 players in EU sim 1 can render that road ''Traffic Jam Simulator'' status.