Primary sources are sources of historical knowledge that were produced by people with direct knowledge of the issue that is being studied. There are not many kinds of primary sources that can tell us about the clothing that the people in Ancient Egypt wore. This is especially true when it comes to the clothing of non-elite Egyptians.
Ancient Egyptians lived, of course, in a time before photographs. They lived in a time before widespread literacy. They also lived quite a long time ago. All of these things mean that there are not many kinds of primary sources about their clothes. We cannot look at family pictures. We cannot look at magazines. We cannot look at diaries that people kept that would refer to what they wore. In addition, because they lived so long ago, we cannot find much in the way of actual remains of their clothing.
Therefore, we have to rely largely on art. We have to rely on statues that they erected. We have to rely on paintings on tombs and other walls. We have to rely on the cases that held mummies. In other words, we have to rely on a few kinds of art that show Ancient Egyptians and the kinds of clothes they wore. This gives us more understanding than we would have about cultures that did not produce art, but there are not enough kinds of primary sources to really give us a completely clear picture of all aspects of how Egyptians dressed.