I wrote briefly about this a few months ago. In short, it's because hieroglyphs were not the script of everyday business. They were an ornate, decorative writing system used on monuments and, in cursive, in religious texts. They are strongly associated with an Egyptian political and religious identity. After the 6th century BC, Egypt was never ruled by its own people for terribly long. There were Persian, Greek, pagan Roman, Christian Roman, and Arabic invaders, all who brought their language and religion. Hieroglyphs thus died out alongside Egyptian religion.