Because of the way the current contracts work - they require some onchain readable count of total amounts, any alternatives would require new contracts and functional wrappers of the underlying. Neither ADA or DOT have a functional/widely traded wrapper with any on chain volume, whilst weth doesn’t work for the market cap weighting calculations, since the cap of weth isn’t the same as that of eth.
Obviously this is something that will need to be thought about in the future, and different/more esoteric and applicable ways of weighting pools can be selected based on the nature of the underlying assets - some standard way to deal with wrappers is needed for instance. But for now it makes more sense for us to focus on what we can actually do without any fudging.