An index can change rather importantly over time. Some segments go through sustained secular performance and become increasingly important on a secular basis. This has happened with the internet relative to much older industries. Sometimes it can be a bubble but for every time someone calls something irrational, there are many cases where something secular is happening.
Below is a chart plotting how much more important Amazon.com is to the performance of the S&P 500 than it used to be. This has come at the expense of names like Exxon.
Another example is Facebook vs Chevron.