it is about the fact that there is only 1 bitcoin which is going against 1700 altcoins. that is why calling it "dominance" is wrong to begin with. if altcoins were good then the real dominance of bitcoin should have been 0.05% instead of forty something! being at forty... just proves that altcoins are indeed shitcoins.
This is a skewed way of looking at it. If you remove the 1650+ coins that obviously trash, as well as the trash coins that somehow made their way in to the top 20 (BCH, TRX, XVG, EOS), things are much more even. It's not fair to combine ETHs share of 20% with a bunch of scam coins.