Why can't you short bitcoins? Part of me suspects it's because a lot of people intend to sell their bitcoins as soon as the market shows a significant downturn

Why can't you short bitcoins? Part of me suspects it's because a lot of people intend to sell their bitcoins as soon as the market shows a significant downturn, and so they don't want to lend their bitcoins in the first place. Although given how large the percentage swings are right now, I have no idea what such a "significant downturn" would have to look like.