People didn’t read back then like we do today.
The average person never, ever learned to read. The average person worked in a field, or in a workshop, produced what they needed to, and paid their taxes (which was whatever the tax collector told them to pay).
Reading was reserved for officials, Priests, and Royals.
So, what. Happens when that land gets conquered and those officials, priests, and royals are no longer in charge?
This is what happened in Egypt.
It first started with Alexander the Great who invaded Egypt in the 300s BC, installing a more Greek-Centric Dynasty there. This dynasty ruled Egypt for 300 years, and while they still used hieroglyphics, Ptolemaic Greek also started to get used/be common.
Then in the first century AD the Romans came knocking, and conquered Egypt themselves.
These romans brought Roman life with them, official writings and laws and letters started to be written in Latin, the language of the new rulers, while hieroglyphics continued to slowly die out. Which it eventually did around the 5th Century AD.
What happened was that the big important people using hieroglyphics all died/were replaced, and the following generations didn’t learn it as much because there were other, more important languages to learn (Greek/Latin).
This doesn’t even include the fact that Latin, having a phonetic alphabet, is arguably just an easier/more convenient language for people to learn, since you don’t have to memorize so many different Glyphs.