Well, the reason datacenters use 48VDC is because that's what central offices used already, so the infrastructure was there, and they made hardware to fit.
The reason for that, though, is some combination of it being a high enough voltage to reach phones a few miles from the CO, without being so high that it would kill people. Now, why 48V instead of 50V... I guess is that it is an even number of lead acid cells (24, in this case). If you've ever been in a exchange, they've usually got the basement full of 48V batteries. (which is why the phone works when the power goes out).