It’s hard to ascribe specific causes and I am definitely not an expert but I can answer a few points of your question. First off, the fading away of hieroglyphics definitely did not have to do with the Islamic conquest. The last known hieroglyphic inscription was written around 400 AD and the Islamic conquest of Egypt did not occur until 642 AD. It seems likely that the Roman conquest of Egypt and its eventual incorporation into the empire as a province in 30 BC played a role as the cultures and writing systems of many parts of the empire were gradually subsumed by Romanization. With that being said hieroglyphics did last for another 400 years so there must have been other factors. I think the rise of Christianity played a major role here as Theodosius outlawed all pagan practices in 393 AD. Christianity had been gaining power in the empire for a very long time but this event may have provided the shove necessary to kill hieroglyphics. The last point worth mentioning was that the empire was in terrible shape at this point in history and a general breakdown in law and order, trade and society itself may have contributed to the loss of hieroglyphics.