Meanings of words change over the years. Words fall out of use, or are reapplied, or new words are invented. The "why" is usually just because the people using the language are spending their time speaking about different things than they used to, and the language adapts to suit their nee More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.