Hashem promised Bnei Yisrael that they would leave Egypt with "klei kesef k'lei zahav u'semalos. " The Ba'al haTurim writes that "smalos" in the plural means exactly two garments -- one to carry dough in, the other the literal shirt on their backs. These clothes did not wear out and lasted miraculously for the 40 year journey through the desert ("simlascha lo balsa mei'alecha...") My wife pointed out that this suggests that they did not change clothes for Shabbos, something we consider very much part of our kiyum of kavod Shabbos.
Perhaps one could explain (derech derush) that we change our clothes l'kavod Shabbos to demonstrate that on Shabbos we feel in the presence of Hashem more than during the week. In the desert, surrounded by the clouds of glory, the man, the well of Miriam, the Jewish people always felt in Hashem's presence -- they wore Shabbos clothes all the time.