I wonder why more people don't use "market" or "market economy" It was the earlier term.
Prior to "Capitalism," the word "market" was used. In 1776, Adam Smith's book "Wealth of Nations," explained economics in terms of a country market so that everyone could understand. However, He then also described wages as being subject to the same forces.(ie labor market) Finally Smith wrote something to the effect that the whole world could be viewed as a market. This is what changed the word market to include a system as opposed just to a location.
ref. "Wealth of Nations"
I have heard that the word "capitalism" became popular about the same time as Karl Marx was writing about it, although I don't think he invented the term. Marx used the term historically for the political control of the state by the wealthy, as opposed to the feudal nobility. The earliest use of the word Capitalism or capitalism (in books) that I could find is 1880!
ref. Google Books Ngram Viewer
books.google.com/ngrams/graph
Graph these case-sensitive comma-separated phrases: Capitalism