Mostly to have a hazmat route around the city (HAZMAT trucks aren’t allowed in the tunnels) and because bridges are cheaper than tunnels. They needed a third crossing because the traffic warranted it.
Most highway tunnels (including the Coentunnel and Zeeburgertunnel on the A10 ring road around Amsterdam) are category C tunnels, which means some hazmat allowed depending on the nature of the materials, the quality of the containment, and the volume transported. Notable exceptions are the Schipholtunnel (category A, fewer restrictions) near the airport and the Arenatunnel (category E, severely restricted) under the stadium.
