Because translations are always culturally biased, and the Septuagint was no different. During the time in which the Septuagint was translated the prevailing notion in the faith was that angels were superior to humans, not unlike our society's understanding of heavenly beings today. Further, there was a prevalent teaching that humans were terribly sinful and tainted and any comparison to God was unthinkable. So, even though the original Hebrew in Psalm 8.5 clearly read elohim , God, the translators took the liberty to use the Greek word angelos , angels, as their translation's choice. Indeed, the King James wrongly translates it this way as well, thus supporting the traditions of the Septuagint as opposed to using correct translation practices.
Every biblical translation reflects the culture in which they are written.
Idioms and metaphors in scripture are often changed to help the reader make sense of the passages. Indeed, there are numerous passages that make little sense to modern readers when translated directly, so translators use words, phrases, and idioms that help the reader understand. In this case, the thought that humans were so highly placed by the psalmist was offensive to society, and so the translators took liberties with the text. In any event, by the time the writer of Hebrews set pen to papyri, the words of Psalm 8 had been significantly changed from the original. So, although this quotation of the psalmist in Hebrews is erroneous today, those of his readers familiar with the Greek translation would instead have applauded his memory.