One reason could be that people are disccovering the use cases for the altcoin and the altcoin is acquiring additional use cases. Utility is important for an altcoin: who wants to buy an altcoin which is useless?
Another reason could be how an altcoin solves a problem older cryptocurrencies have. Many altcoins based on Ethereum allow for faster transaction processing and cheaper gas fees. Some altcoins act to make Bitcoin transactions more private. Then here is Hive and both social interaction as well as the attention economy.
When cryptocurrencies become more established across the board, they won't rely so much on what happens to Bitcoin or Ethereum or even Dogecoin.