Well, very good aquestion, and I've been thinking a lot about it recently.
During 2017 I invested my funds in bitcoin and altcoins in the same time.
When bitcoin grew, altcoini was growing even more but when the bitcoin began to fall altcoins have lost even more value.
When the crypto market began to recover, Bitcoin partially recovered but my altcoins have never recovered.
Why?
I guess because everybody know bitcoin and it's main entry point in the crypto market.
Everybody wants to have some bitcoin, it's crypto market leader but we have so many altcoins that people are confuse which one is good choice.