The voltage is negative to ground because that polarity was found to minimize electrolytic corrosion. -48V (usually closer to -52V in practice) because it was provided by a string of lead-acid batteries (~2.2V each when charged, close to 2V when “empty”) so the voltage had to be a multiple of 2, and the “old” telco system was constant-current, so starting with a higher voltage made it easy to accommodate both nearby subscribers (low resistance lines) and distant subscribers (higher resistance lines) simply by adding more resistance to the short loops.
Later, this was done automatically by incorporating various sorts of “constant current” devices into the terminal gear.
The other reason is that the batteries over the years were produced in multiples of 6 VDC (6, 12, 24, and 48 Volts).As for the magnitude (48 vs 24 or 72) - and 48v is a compromise between safety and power enough to go the distance.
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