Why we using positive as ground in telecom field?

Answer 1

One way or another, when you apply a voltage across two conductors you will start some sort of electrochemical reaction. With respect to a telco connection, you have two choices: lose some of the conductor or gain some corrosion. The telcos have found that corrosion is the most difficult thing to deal with, so they opted for losing some copper.

The telcos use a positive ground for the same reason the pipeline industry uses what they call cathodic protection (See reference 2). There is a also a good telco reference (See reference 1).

Answer 2

If a negative potential were applied to the shields and grounds (which are actually connected to earth) of copper and steel jacketed cables, an electrochemical corrosion occurs in the cables. By making the ground potential positive (with respect to "everything else"), that corrosion is greatly reduced or eliminated alogether.

"Ma Bell" figured this out a long time ago: negative ground = corroded cables (very rapid corrosion), positive ground = very little corrosion.
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Answer 3

To disperse the static.
Which is not good for communication