The Martian Atmosphere Is Unbreathable!

in a story in the original book (missed out in this dramatisation) during the early colinisation of mars a "johnny Appleseed" character travels Mars planting trees to give it a breathable atmosphere.

Even without this though you have to remmeber that the book was written in the 1940's when no one had any idea whether or not martian atmosphere would be breathable. Taken in that context it is not a "Blooper"

As recent as the late 1960s, scientists still knew very little about Mars. No country had space probes that orbited Mars or landed on the surface (at least successfully) until 1977.

Back in the late 1960s, it was actually theorized that the Martian atmosphere might be just...just...barely dense enough for prospective astronauts to forgo bulky, hermetically-sealed moon-type spacesuits. Optimistic scientists hoped Mars astronauts would be able to wear less bulky suits and might actually be able to walk around the Mars surface with just oxygen breathers, sort of, like divers' aqualungs.

Well, then came the 1977 first successful Mars surface probe lander, from the USA. Then, disappointment. The Mars atmosphere was far, far, too thin. It is 95% carbon dioxide. Later surface probes in the later 2000s revealed, beyond the already known iron oxide dust, that the surface contains a lot of other metallic and mineral deposits, when mixed together, make up a nasty toxic combination so that future Mars astronauts had better suit back up in those bulky Moon spacesuits.

TRIVIA: Two-thirds of all Mars space probes sent out by Earth have failed or disappeared, leading to a universal joke among the world's space agencies that Mars is a 'black hole' or 'Martian Bermuda Triangle' for space probes. There has been massive incredible bad luck with Mars space probes, experienced even by the U.S. The Soviets actually landed two probes on the Mars surface, circa 1971-1973 and then, nothing, no signal, nothing. The movie, Mission Mars, actually uses one of those probes as a means for a stranded U.S. astronaut to obtain a still-functioning battery and a still functioning, built-in Soviet rocket meant for catapulting rock samples back into space for retrieval.

The Martians are just tricking everyone into thinking Mars atmosphere is unbreathable so we will leave them alone a bit longer.