[Occasionally6]

Definitely cost. Back when most of the Japanese manufacturers had systems, it was used as a marketing tool. Often the cars would have decals on the bottom of the doors with all the acronyms - ABS, 4WS, 4WD - for the features so the customer could be reminded of where the money went.

There were various ways to achieve the rear wheel steering. Some used hydraulics but Honda's was a purely mechanical system with an additional shaft to the back of the car. That operated a worm and peg mechanism. At large steering angles the rear wheels steered in the opposite direction to the fronts, to decrease the turning circle. At small steering angles the rear wheels steered in the same direction as the fronts to improve grip.

The hydraulic systems could be speed based. Some only steered the rear wheels in the same direction as the fronts, some also did the rear wheels out trick.

The now unheralded passive systems rely on the deflection of the (quite sophisticated in design) suspension bushes under cornering forces to alter the angles of the rear wheels.