The start of the first book hints about why:

Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...

It appears that the planet-like computer was been placed in an obscure and untrafficked part of the Milky Way galaxy. From this we might infer that they simply did not expect anyone from the outside to come upon it, and therefore left it to its own devices. This is careless and sloppy thinking of course but nonetheless not impossible.

Also do note that in the narrative the Golgafrinchans caused the demise of the original cavemen of Earth fairly quickly, the latter starting to die out within only a year of the former's arrival, which is hardly even a shake of a lamb's tail in the ten million years the program was running. So even if the mice were keeping an eye on it, unless they did so meticulously and with utmost scrutiny then it is not hard to understand how this glitch in the input data could have gone unnoticed.

The start of the first book hints about why:

Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...

It appears that the planet-like computer was placed in an obscure and untrafficked part of the Milky Way galaxy. From this we might infer that they simply did not expect anyone from the outside to come upon it, and therefore left it to its own devices. This is careless and sloppy thinking of course but nonetheless not impossible.

Also do note that in the narrative the Golgafrinchans caused the demise of the original cavemen of Earth fairly quickly, the latter starting to die out within only a year of the former's arrival, which is hardly even a shake of a lamb's tail in the ten million years the program was running. So even if the mice were keeping an eye on it, unless they did so meticulously and with utmost scrutiny then it is not hard to understand how this glitch in the input data could have gone unnoticed.

The start of the first book hints about why:

Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...

It appears that the planet-like computer was been placed in an obscure and untrafficed part of the Milky-Way galaxy. From this we might infer that they simply did not expect anyone from the outside to come upon it, and therefore left it to its own devices. This is careless and sloppy thinking of course but none the less not impossible.

Also do note that in the narrative the Golgafrinchams caused the demise of the original cavemen of Earth fairly quickly, the latter starting to die out within only a year of the formers' arrival, which is hardly even a shake of a lamb's tail in the ten million years the program was running. So even if the mice were keeping an eye on it, unless they did so meticulously and with utmost scrutiny then it is not hard to understand how this glitch in the input data could have gone unnoticed.

The start of the first book hints about why:

Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...

It appears that the planet-like computer was been placed in an obscure and untrafficked part of the Milky Way galaxy. From this we might infer that they simply did not expect anyone from the outside to come upon it, and therefore left it to its own devices. This is careless and sloppy thinking of course but nonetheless not impossible.

Also do note that in the narrative the Golgafrinchans caused the demise of the original cavemen of Earth fairly quickly, the latter starting to die out within only a year of the former's arrival, which is hardly even a shake of a lamb's tail in the ten million years the program was running. So even if the mice were keeping an eye on it, unless they did so meticulously and with utmost scrutiny then it is not hard to understand how this glitch in the input data could have gone unnoticed.