Because people fear of missing out when bitcoin rises, and fear when bitcoin drops. People get completely out of bitcoin when it drops, and that panic affects alts as well. The price dropping makes people lose confidence in crypto in general, thus more money leaves because they can't handle the pressure. When bitcoin rises, people want to get in on that rise so they move into Bitcoin instead to get those gains. Alts generally pay out better short term, so I don't really understand why more people don't take alt positions, but I'd still attribute it to fear overall. People really don't want to miss that train, so they get on whatever one is moving.