IIRC, the rule was established because some bad monks, used vehicle pulled by animal or (female) slaves.
This was criticized, because that is the behavior of the rich, also it exploits other living beings.
At that time the monks from the group of six traveled in vehicles, sometimes pulled by a female animal with a man driving, at other times pulled by a male animal with a woman driving. People complained and criticized them, âItâs as if theyâre at the Ganges festival.â They told the Buddha.
âYou should not travel in a vehicle. If you do, you commit an offense of wrong conduct.â
The group of six, is the infamous group of bad monks who caused all sort of rules to be established.
Based on the origin of the rule, vehicle can be used if it is not unreasonably luxurious, and not exploiting other living beings. The rule already has precedent of being relaxed.
Soon afterwards a monk who was traveling through the Kosalan country on his way to visit the Buddha at SÄvatthÄ« became sick. He stepped off the path and sat down at the foot of a tree. People saw him and said to him, âVenerable, where are you going?â
âIâm going to SÄvatthÄ« to visit the Buddha.â
âPlease come with us.â
âI canât. Iâm sick.â
âThen please come inside the vehicle.â
âThank you, but the Buddha has prohibited us from traveling in vehicles.â Because he was afraid of wrongdoing, he did not accept. When he arrived at SÄvatthÄ«, he told the monks what had happened. They in turn told the Buddha, who said,
âI allow a vehicle when youâre sick.â
The monks thought, âPulled by a female or by a male?â
âI allow a rickshaw pulled by men.â
Soon afterwards a certain monk was even more uncomfortable when jolted around in a vehicle.
âI allow a palanquin and a litter.â
https://suttacentral.net/pli-tv-kd5/en/brahmali?layout=plain&reference=none¬es=none&highlight=false&script=latin