Why -48V is preferred?
(i) Positive voltages cause comparatively more corrosion in metal than negative voltages.
(ii) Negative voltages are safer for human body while doing Telecom activities.
(iii) Lightning may cause positive voltages in the equipment circuitry. In that case, negative voltages (lack of electrons) neutralize positive charges and prevent excessive heat.
(iv) Negative voltage is safer for long telephone line for transmitting power trough it.
The reson for electing -48V in reference to ground is to avoid the nasty galvanic effects in telephone cabling. +48V would have nastier effect on wet cabling.