Because altcoin owners turn to btc. They want to earn extra profit by selling their acquired altcoins against btc. In such cases, btc first moves forward, then altcoins follow.
that looks similar to 2017's movement. I have faith in my bitcoin and im too late to convert all of my altcoin to the bitcoin. it looks like the market is having a lot of support from the latest news. More big companies, stock market, and governments are starting to put their attention in the cryptocurrency too.
In a few years and the price of bitcoin will grow a lot. Im ready to get on the train. Altcoin will follow after bitcoin has done with its pump.
The movement is really have something in common with last 2017 pumped, slow in terms of big growth but much better than a pure hype without any supporting grounds, if you research well then you can have the confidence to buy more and hold, waiting for the market to be fully uptrends movements.
This is just a temporary situations inside the Alts community, bitcoin is just starting and after that alts will follow.