IN A NUTSHELL: Because meteoroids capable of piercing an astronaut’s multilayer spacesuit or a spacecraft are actually incredibly rare. The suits and the vehicles had protective layers designed to absorb the impact of the minute specks that constitute the vast majority of meteoroids. There is no protection against larger meteoroids other than the very low probability of being struck, but this is an acceptable risk, as demonstrated by the fact that satellites, space probes, crewed spacecraft and the International Space Station don’t get riddled by meteoroids.
THE DETAILS: The Moon is pock-marked with craters produced by meteoroids: rocky or metallic masses of all sizes that travel through space and can strike the Moon at speeds up to 80,000 kilometers per hour (about 50,000 mph). Although the terms meteor and meteorite are often used, strictly speaking a space rock is a meteoroid when it travels through space: it becomes a meteor only if it punches into the atmosphere of a planet or moon and forms an incandescent trail and then becomes a meteorite if it reaches the surface of the planet or moon instead of disintegrating completely.
Looking at the Moon, it’s understandable that someone might wonder how the moonwalkers coped with this constant lethal danger. The answer is actually quite simple: they relied on probability. Meteoroid showers aren’t as frequent and dense as often depicted in Hollywood sci-fi productions. If they were, our fleets of satellites that provide us with weather data, TV programs and telephone calls would be destroyed all the time and the International Space Station would be Swiss cheese after over a decade in space. Several automatic probes have been traveling through deep space for three decades or more and have survived essentially unscathed.
Actually, most meteoroids are literally microscopic in size. They have an enormous speed, but an almost negligible mass, so if a micrometeoroid strikes an astronaut it is stopped by the spacesuit’s outer layer, which is designed for this purpose. The space suits used by the Apollo moonwalkers and the ones used today for work in space have essentially the same type of multilayer protection against micrometeoroids. That’s one of the reasons why they’re so bulky.
Non-microscopic meteoroids are quite rare. While the Moon’s cratered surface might appear to suggest otherwise, one must bear in mind that those craters are the result of millions of years of exposure. Apollo astronauts and all lunar spacecraft (including the Russian Lunokhod rovers) had a vanishingly small chance of being struck by a significant space pebble.