Because in a market, once bitcoin or etherum is bearish, definitely all tokens suffer decrease in prices
Which means that altcoins can increase or decrease without affecting etherum or bitcoin prices
But altcoins suffer or gain when the trade pairs (bitcoin or etherum ) reduces or increases respectively
Bitcoin has a dominance of over 50% in the market, and that is what they should be. Altcoins can also move according to their innovations, but it does not take long to get back on the trend.