SDI-12 sensors usually use 12 volts for sensor excitation because that’s what the specification for the protocol dictates. Fun fact: the “12” part of “SDI-12” does not refer to the voltage, but rather the baud rate, so the name actually means “serial-digital interface at 1200 baud”). Back in the days when the protocol was first being used, most sensor manufacturers used 12 volts for their sensor power. Over the decades since, changes in technology resulted in sensors that can use lower excitation power, or sensors that can regulate their own power if you give them a wide input range.
There’s lots of interesting history about the standard on the SDI-12 Support Group website: https://www.sdi-12.org/history
Plus there’s helpful technical details about the protocol in the full specification sheet: https://www.sdi-12.org/specification
I have personally used SDI-12 sensors that operate at 3.3v, 5v, 9v, 12v, so it really depends on the manufacturer’s designs and the sensors unique power requirements that dictate what excitation voltage must be supplied to the sensor. In general, they’re supposed to all be able to be powered by 12 volts in order to meet the requirement of the SDI-12 standard. But some manufactures have added the special feature of running from a lower voltage since nowadays many loggers operate on smaller battery supplies.