Is it true that pre-jet air planes used to fly much lower? If so, why?

Among many factors allowing a plane to fly in higher altitudes, two are more prominent. Very briefly:

• Pressurized cabin

• Engine performance and its oxygen-breathing needs.

Both of them apply to jetliners which have pressurized cabins and because their jet's compressors increase the input air pressure. Although the turbo compressors or other features can help to provide oxygen to the piston engines to some limit.

The Jet engines actually take advantage of the higher altitudes' lower temperatures and air pressure as well. Because their engine operated under very high temperatures. When the incoming air is cold and of lower pressure it will not get as hot being pressurized and burned before the turbine, protecting the engine from excess heat.

TL:DR 1. Yes, 2. Because of the differences in engine performance wrt flight speed. 3. You mostly can avoid weather at 35,000 feet.

Airplanes potentially have better fuel efficiency at higher altitudes simply because there is less air resistance the higher you go. The problem is that there is less lift if there is less air, so you have to go faster. Ironically, the faster the plane can cruise, the lower it's drag becomes for the trip, because it can sustain flight in thinner air.

So it becomes a matter of engine performance at speed and altitude. The typical cruising speed of the Pan Am Clipper was 155 mph (1942). It might top out at 15,000 feet after 12 hours or so, but this wasn't a good idea due to the lack of cabin pressurization.

Post war piston commercial aircraft such as the DC-6, DC-7, and Constellation had service ceilings of between 24,000 and 28,000 feet. The Comet 1 (1949) was designed for 36,000 feet. The jet engines of the time had horrible fuel efficiency, and they just worked better at altitude and speed. Piston engines, even with multispeed superchargers and turbos, still lost power above 15,000 ft. Commercial radial engines never really benefited from ram air induction or the Meredith effect. Their induction systems were always a plumbers nightmare.

DC-6 cruise speed 315 mph.
DC-7 cruise speed 346 mph (1956)
Powerplant: 4 × Wright R-3350-988TC18EA1-2 18-cylinder turbo-compound air-cooled radial piston engines
3,400 hp (2,500 kW) each for take-off at sea level
2,800 hp (2,100 kW) in low blower at sea level
2,850 hp (2,130 kW) in low blower at 4,300 ft (1,300 m)
1,900 hp (1,400 kW) maximum cruise in low blower at 14,800 ft (4,500 m)
1,800 hp (1,300 kW) maximum cruise in high blower at 24,400 ft (7,400 m)
Boeing 707 cruise speed 525mph (1958).

So it really all comes down to developing faster planes. Speed buys you more altitude.

Modern passenger aircraft are flying ever slower. They are basically constrained below 40,000 feet now, and engines are being developed around that ceiling for maximum fuel efficiency.