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:

* https://en.wikipedia.org/wiki/Mutual_fund

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):

* https://en.wikipedia.org/wiki/S%26P_500_Index#History

Then there had to be interest from investors. So it's not entirely surprising that it took until the 1970s with Vanguard to really create something.