Because some exchanges have the potential to disappear after a little price moon.
In previous years some exchange already shut down because of bankruptcy or exit scam, After they raised a decent amount on IEO they run with investor's money. So we need to be concerned about what exchanges we are trading on they can run anytime with our money only trust reputable exchanges like Binance. I trust them personally because they have insurance in case of hacking. But to be safe withdraw your coins into your wallet if you are planning to HODL.