From what I've read millet was one of the Romans' major crops. However, these days, it's primarily used as animal feed, and is unknown in many European cuisines (mainly used in south-east Europe, and even then not to a great degree). Why did this decline in the popularity of millet as human food happen?
Hi! Hopefully someone expert on dietary starches (hmm.. /u/agentdcf?) can help out, but there was an old post on millet that may provide a couple of leads worth further investigation
Sorry, I've no idea. I have seen references to it being grown much later than the Romans; for example, Natalie Zemon Davis's famous study of early modern peasant life in southwest France, The Return of Martin Guerre, does mention that peasants in that region grew millet among other things.
It's tempting to suggest that it doesn't make very good bread, and therefore gave way to the bread-making grains of barley, rye, and especially wheat. But, why people would prefer bread to more porridge-like or other preparations is not necessarily clear. So, that's not much of an explanation.