It's a bit of an arbitrary determination; 60V is the international common denominator on what various legal or civic (e.g. insurance) regulations consider safe; https://en.wikipedia.org/wiki/Extra-low_voltage#Separated_or...

48V is somewhat of a dinosaur from the days when you designed things with a 20% margin since that's what was cheaply attainable. Regular PoE is 54V these days (10% margin), and some more specific applications creep even closer to 60V. (I've seen 57V and 58.5V, which is of course 5% and 2.5%)