Why -48V power supply are used in Telecom?

Positive voltage cause comparatively more corrosion in metal then Negative voltage.

It prevents electrochemical reactions from destroying buried copper cables and rendering them useless if they happen to get wet. Negative voltage also protects against sulfation on battery terminals.

Negative voltage is safer for human body while doing Telecom activities.

Today it is generally accepted by safety regulations and electrical code that anything operating at or below 50V DC is a safe low-voltage circuit.

Thundering may cause positive voltage in the equipment circuit. In the case negative voltage (Lack of electrons) neutralizes positive charges and protects producing heat.

Negative voltage is safer for long telephone line for transmitting power through it.

A standard lead-acid battery provides 6V. This number corresponds to 8 of those, which is pretty standard. It's the highest, safest compromise voltage to run over long wires. Negative voltage is used so that leakage currents to ground caused by moisture do not electroplate away the copper in the wires.

The power supplies for base stations mainly employ the rectification power supply, and most base stations employ -48V rectification power supply equipment except for some equipment like Ericsson.

The rectifier circuit takes alternating current (AC)and converts it into a positive direct current (DC) output. The rectifier is to provide desired DC power for the application.

As a matter of fact, -48VDC is the standard in communications facilities serving up both wired and wireless services, including cell towers, local cable TV vaults, and legacy central offices of the various incumbent local exchange carriers (ILECS).