Other people already mentioned safety. But you can actually go to 96V if needed by running a cable with -48V if you need extra power.

One another advantage of the new 48V architecture is that it doesn't depend on the car body for the current return path. This opens up possibilities of adding sensors that detect current leakage, to pinpoint areas with defective wiring and/or components.

Depending on the country you live in, the laws might allow you to do work on equipment that is below 50 volts, but require you to be a certified electrician for anything above that.

I believe 50V is a safety limit.

Below 50v is considered "low voltage". Higher voltage would require different safety considerations.

And that's because of your skin resistance. Around 50V is when voltage starts to overcome it.

usually. It's very possible to get a shock off of significantly lower voltages in bad conditions (very sweaty skin, for example).

At some point, safety.

A short/failure at 100V is much more dangerous than at 50V. Both from a fire-safety perspective as well as an electrocution risk perspective.

48v is probably more common than 96v in general, so more components available already.

As mentioned in another comment, it’s also close to the safety limit for low voltage systems.

IMO solar pioneered (in recent history) 48V DC systems, which is an easy multiple of 12V to stay below the 50V “high voltage” safety threshold.

It allowed people to use smaller gauge wire and chain together multiple 12V batteries that are readily available.

Telephony has been using -48V as a line voltage for a very long time, probably > 100 years.

Is that for power deliver, or signaling?

-48VDC was standard for powering telephone equipment, and is also common standard for direct current datacenter connections.

Signaling on POTS easily hit over 100V, btw.

I remember getting zapped when the phone rang when I was a teenager back in the 70's. Good times.

Both. It powers old school telephones.

For which - why standardize around this connector and not XLR, which is the first thing that comes to mind for 48V?

Too big/bulky?

The phantom power at 48 volts used with XLR connectors only have a current at about 10 milliampere. Enough to supply power to a little microphone.

The connector is bulky and of metal, and designed to be used inside. It's also expensive compared to other connectors. There are a lot of cheaper, more suitable connectors, designed to carry power.