From the Sfas Emes (translated & paraphrased by your host):
How can the wise child ask for the reason behind a Biblically ordained law? It does say in the Bible, "He gives his sayings to Jacob, his laws and ordinances to Israel." There are cases in which one does not understand the reason behind a practice, but by the very practice itself one comes to know the reason nevertheless. Thus is matzah without a taste [טעם]* - so that it can fade into the background, leaving the eating of the matzah itself as the reason. Similarly, the last food eaten at the Seder should be matzah.
*This word can also mean "reason." - ZB