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.
Because they think that bitcoin was is the safe zone of investing rather than other altcoins, and they switch their altcoin to bitcoin.
The market makes worst in bitcoin price because of the dollar when the trader wants to exit and convert all of the funds he/she had then bitcoin is really affected. That's why bitcoin now is regaining dominance and it was obviously right because altcoin depends on bitcoin price.