Why was the word Hades used in the Septuagint instead of translating the original Hebrew word for Sheol to Greek?

In addition to the answers from Elke Weiss and Niko Vasileas, note the wrinkle thrown up by Sheol: there is some evidence that Sheol is anthropomorphic in the Hebrew Scriptures, with a womb, a hand, and a mouth.

So Sheol corresponds to the House of Hades, the place (as indeed is the Christian usage of Hades: Christian views on Hades). But the Septuagint translators might have had motivation to map Sheol to Hades himself.