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.
I guess the developers of the altcoins aren't as perseverant as the developers of bitcoin are. All the altcoin developers do good in the start and then if the altcoin doesn't perform well then they lack interest in that project and try to find another project to make some money.
Also as the price of bitcoins is increasing but the Altcoins are at the same price or many of them are in negative so people are loosing their interest in altcoins and are selling their alts to buy bitcoins.