Nice fact sir, I guess its because bitcoin owners invested their bitcoin from buying altcoins. One thing for sure one day ETH is the next bitcoin.
Bitcoin has been steadily losing market cap dominance, from 90%+ just two years ago to ~80% today.
When it comes to exchange dominance it's just 66.4%.
This trend will probably continue and in a few years there will be people using cryptocurrencies that do not know what bitcoin is.