This cryptocurrency index fund has not been created to have the very best 20 coins of the market and keep them unchanged forever. If you have a look on the website (www.bittwenty.com), you will see the explanation on how the algorithm selects the coins.
They are basically selected by market cap, the top 20. Then they are calibrated so BTC, ETH and other big market cap don't have too much weight in the balance.
Every month, there is a revision like the one described in this blog. 20 new coins are selected, most of the previous one remain off course.
We consider that if the project has a high market cap, it deserves its place in the index. I also believe that these won't be the best 20 coins forever but it's not a problem because every month they are renewed.
Bittwenty will always have the best coins of the moment and follow the economic growth of the cryptocurrency sphere in a global way.