@bharat, Interesting question about why there is usually more postcursor than precursor.
First of all, I think mathematically it is always possible to come up a channel which has more precursor. But for a physical cable, I guess one qualitative explanation can be as following:
There are two main loss mechanism in a cable channel: skin effect and dielectric loss. The first one causes only post cursor and the second one causes almost symmetric precursor and post cursor. The overall loss is convolution of the two losses. Therefore, qualitatively, more postcursor exists in physical cable.
A good explanation about these two loss mechanism is covered in this paper:
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1610643 Of course, this explanation is still hand-waving. Hope some expert can give more intuitive answer.