Why not just bolt it underneath and put a cover over it? The cover would weigh less than a couple of kilos and probably improve the vehicle's overall aerodynamics anyway.
So I put it down to a packaging issue. They had two choices and under the bed solution provided the extra benefit of being able to use that space for other purposes. Many newer vehicles do not carry spare tires and this trend has been accelerating as tire technology has improved greatly and road side assistance is common that people rarely if ever have to change a tire.
Not sure if I would be so quick to do it in a modern car with alloy wheels and an even mass distribution but the car in question was a tough old Volvo Amazon with steel wheels and the flat was on a back wheel; in the Amazon most of the weight of the car is over the front wheels so the flat really didn't stress the wheel much.
That was about thirty five years ago.
Well, if it is bolted under, it needs its own cover/plate, which increases the weight of the surrounding structure (since whatever's there can't be structural).