Because basically not all altcoins follow the bitcoin price trend. All crypto users love bitcoin, but not all crypto users like all types of altcoins.
They have different interests for other altcoins, this is the reason why prices are not always in tune with bitcoin.
Yeah it seems like a good reason why alcoins price is not following the uptrend price of bitcoin. And beside of that I think alcoins price is supposed to be rising a little bit slower from bitcoin because all of the investment at first will be targeting on bitcoin and then for altcoins.