Why is tennis net lower in middle?

The net is lower in the middle to create a tension downwards on the cable running through the top of the net. In the past this cable has broken and due to the tension within it has whipped to one side, which can result in serious injury to anyone caught in its path. The tension downwards pulls the cable and the centre down and if it does break it will first be brought to ground and is less dangerous.

Lawn tennis is also a follow on sport from Real Tennis or Royal Tennis. In that sport the upper cable is not under high tension and therefore sags in the centre. The balls used in Real Tennis are hard like a cricket or Baseball and does not need high tension in that cable. A lawn tennis ball is much lighter and was the reason for increasing the tension on the cable. Bringing the cable down in the middle also goes someway to duplicating the original lower section of the net.