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.
I think the reason to buy when the price of Bitcoin reduced is the hoping of the investors that the price of BTC will be rising up to make profit. So that they willing to buy a lot of bitcoin and hold for the long-term to hope the bull-run of the market will occur.