Because anyone telling you they're "supposed to gain" doesn't know what they're talking about. When the price was $19,000, people were still saying it was a buying opportunity because the coins were "supposed to gain" all the way to the moon. But they didn't, because that's not how markets work. People wouldn't know the moon if they crashed right into the side of it, which we very likely did. We already went to the moon, and now we're on our way back down to Earth in a slow, controlled descent.
The market most certainly did crash. It's down over 70% from ATH. That's a monster crash. Any momentum to try and break this fall is short lived and easily reversed. As for why you would choose to sell, it's because you don't know how low it's going to go, if it will ever recover, or if it does recover how much the recovery will be or how long it will sustain. You would sell to limit risk of additional losses.