This index is being calculated since 2013 so when it started it was based on coins that were traded with large volumes on BTC-e and Cryptsy.
But price absolute values are extremely different so you need some coefficients.
Besides some coins are more important (Litecoin, Namecoin...) so more weight in % they have.
If traded on BTC-e and also on Cryptsy - more weight.
And % were fixed at the beginning. It was necessary to calculate ratios. That's about ALT19 index.
And there is also ALT100 index where 100 coins included with 1% weight each.
Ok, coefficient is clear but why is it fixed if You claim that initial basing is on large volume? What if volume changes? How will You adjust for %?
The general idea of indexes: to compare, to see trend history relative to the value at the beginning.
If we include new and new coins or change weights every time, there will be no stability. You see how new coins are dumping altcoin market now.
Using Indexes means dealing only with fixed number of coins included.
About % weight: there may be some corrections in future if things change.