Because it is at the reduced rate like this that you can buy and make profit by selling when it rises. Or do you prefer to buy when it reaches it's al time high where you probably can't purchase it. This is the perfect time to invest into bitcoin and wait for its bull run hopefully ending of this year.
Bitcoin is not affordable when it has high price on the market unless you have big amount to put on it that is why many are gaining an opportunity when it reduces the prices because good profit could get when it bounce back. Many wait this moment that it is lower because huge volume can get in cheaper amount.