It was also difficult to implement in a practical way for most people. While mutual funds (SICAV in EU, OEIC in UK) have technically been around since stocks have been around, implementing an index-tracking fund needed (a) indexes to be invented, and (b) computers to be able to do data processing:
Perhaps the Dow Jones was small enough of an index to keep track of manually, but things like the S&P 500 would have been harder (never mind the Russell 3000 or total market):
