Why is it that car audio is usually 4 or 2 ohm, and home audio 8 (or higher)?
Traditionally because of available voltage.
Of course it is possible to use D9CDC converters, which all > 20W/4ohm car stereo systems have) with 12V (actually more like 14V) and bridged operation one can make 9Vrms.
This will give 2x9W in 4ohm or 4x9W in 4ohm (front + rear) and up to 4x18W in 2ohm.
When allowing say 10% distortion one can even quote 20W or 22W in 2 ohm/channel.
In 8 ohm it would only be 4x4.5W.
So the main reason is you can provide more power in 2ohm than 8ohm from the same power supply rail voltage.
In those + 100W amps internal rail voltages are increased and voltage rail limits aren't an issue.
Current does become an issue though. 400W in 12V requires about 40A which requires very thick wires at 12V.