Because alt coins have no value outside of their link to bitcoin.
Look at coins like Ludena Protocol though that have their own internal functional economy and do not rely on the price of bitcoin. these are the types of projects that wil succeed in the next altseason
I would say this for 99% of altcoins, but not all. I mean, some coins are even so unique that Bitcoin has a link to them in the opposite direction,,, for example Litecoin, which is always the testing ground for things that Bitcoin is not ready to do yet!