If you want to estimate the stock market, would you rather:

1. sum 500 of the biggest companies by size (price * n shares).

or

2. have WSJ editors select 30 companies by any criteria they see fit, but you don't get to see the size of the companies, only the share price.

Note that the DJIA also changes constantly over time (as discussed elsewhere in this thread), as do the weights assigned to the individual companies constituting it.

The way that the DJIA changes isn't the same as an index of, say, the n most highly capitalised equities might (Fortune 5, 10, 20, S&P 500, etc.).