scudpilot,
I'm oversimplifying, probably.
The aerodynamic forces on the wing structure tend to twist it upwards and backwards. So you have to design for a stiff enough wing with a 'clean' wing (think VC10).
Hang a few engines under the wing, well forward, and you compensate for that 'twist', so you end up with a lighter wing structure design.
It's only for small jet liners (RJs) and business jets where that doesn't really work because the engines would be practically dragging the runway, unless you increase the size of the landing gear significantly. So there it makes more sense moving the engines to the back.
CJ