The main reason is the rear engine configuration.

In a front engine vehicle, when you are moving, the wind will blow any fuel odor into the drivers compartment.  Any fuel leak, even the most minute will be detected immediately and the driver or passenger will do something about it.  This is not the case of a rear engine car, especially a van as long as the Vanagon.  Slight drips and weeping leaks will not be detected when the van is moving.  Many leaks will stop when the engine stops and the fuel is not longer under the 85psi pressure the fuel injection system subjects the tubing and connectors to. So, in time, the leak gets worst and worst till it drips into the hot exhaust and catches fire.

Yau-Man