The picture above is an excellent illustration of the main reason.
That must be a very strong structure to carry those engines and transfer their thrust to the airframe. And so must add considerable weight.
Why not hang the engines from a horizontal structure that you already have i.e. the wings? And then indeed (as has already been mentioned) the wings need less strength because the lift that overcomes the weight of the engines does not need to be transferred to the airframe. The engines are also very close to their fuel supply.
One begins to wonder how rear mounting designs survived as long as they did.
Of course for business jets there isn't sufficient clearance under the wing so they continue with the rear mount approach. Except I seem to recall seeing a picture of a light jet (Honda?) with top mounted wing pods! Maybe the future?