Power over Ethernet: AC?
The reason is some of the exiting PoE schemes use the communication wires as power wires, too.

See e.g. https://en.wikipedia.org/wiki/Power_over_Ethernet#Pinouts

Transformers are used to avoid galvanic connection between ethernet devices, and this also comes in handy for PoE, as DC power cannot pass these transformers. AC would pass and required additional circuitry to separate power from signals.