Simply put, there are not enough transactions to completely saturate every block at the moment. Even when the mempool is completely clogged up, you can still get empty blocks if there is a very short interval between two blocks as ranochigo pointed out. But at the moment, even when there is a longer block time then blocks might not be full. The last block for example, 692,415, was mined 7 minutes after the preceeding block but was still only ~400 vKB, or 40% full.
The last block only had 48 transactions!
Using number of transactions is a poor way to judge block fullness - better to use virtual kilobytes or megabytes (vKB/vMB) or mega weight units (MWU). 6 blocks ago, block 692,410 only had 147 transactions and yet was completely full at 1 vMB, whereas the latest block I mentioned above had 1057 transactions (7.2x as many) despite only being 40% full.
or that people are yet to potentially panic sell if ever a bigger drop comes.
Or maybe people have already panic sold, and that's what dropped us to sub-$30k levels. It would seem most newbies experience a drop of >5% and they panic and sell immediately. There won't be many newbies who bought in at $60k still holding at $30k, I would have thought. Maybe the majority of people holding now are the people like me who will hold until $1.