Most altcoins are created for a short period for a fast gain. Look at the coinmarketcap site, you will not recognise half of the coins there = fast turn-over. However, a little bit solid altcoins like ethereum and bcash dropped hugely so I think people are escaping to the safest alternative which is bitcoin.
many have already bought assets with very large capital when bitcoin has very low prices, and the more popular bitcoin is that many believe it and already believe in bitcoin, therefore bitcoin still dominates even though prices are very low.