Because I have just read that the fork takes place in about 118 more blocks (less than 19 hours) and ATH was two days ago..
People will sell their BTG to buy more bitcoin and there are many countries and companies that will use bitcoin in the future so do not worry about the price.
I'll talk about something that shocked me a short time ago, BTG started being traded on yobit and the price was 0.03, in a few minutes it went up to 0.19 and until the people of yobit chat said: "time to sell, now to sell .. . "The price dropped in a few seconds to 0.08. wow if BTG were a currency used by some government or some bank had invested in BTG? Yes, altcoins are dangerous.