Because some of alts is depending on the bitcoin price when the bitcoin price is decreasing alt price will be increase rapidly when they already saw that bitcoin price is pumped they will sold their alts to be sure that they will still earned profit. And if im going to choose which coins i will be choosing i'll always choose bitcoin.
Choice is upto yours. Even altcoins may increase in price within some years but not compare to bitcoins however you may get the profit by that if you are a long term bitcoin holder stick to bitcoin itself. Sure you will be a millionaire within 10 years, It may 10000$ worth at that time.