Because the market is correcting. If you go back and look at the BTC dominance pre-2016 you will see ~80% dominance for BTC used to be the norm. Now that alt coin pump is dying down things are going back to the way they were pre-2016.
that is true but it was not all because of the altcoin pumps but also a big percentage of it was because of the high rate of new coin creations. i remember back in 2017 there were days that we saw multiple new tokens appear out of nowhere and each had unbelievably huge supplies! they were practically creating millions of market cap out of thin air and in a matter of seconds.
so obviously as they are dying they are taking that fake market cap out with their death too.