The truth is simple - altcoins were in a bubble, much bigger bubble than Bitcoin was, realistically there's no way more than a dozen of coins can co-exist - the point of money is to be universal, so just like the US dollar is welcomed everywhere in the world, Bitcoin is the king of crypto. Tons of altcoins were claiming that they will dethrone Bitcoin, but when investors started realizing that it was a lie, those coins started losing their value.