it is partly because people generally have no faith in altcoins so they never stick around in them for long. and partly because everyone who has been around long enough will want to increase the amount of bitcoin they have so they al ways take the opportunity to buy more of it before its price goes up. that is why altcoins are abandoned like this.

and finally it is because this trend has been happening for as long as this market existed so everyone is now automatically dumps altcoins without thinking about it.