This is my conjecture:

I suspect that this is due to the TRON integration. In the original design for SteemIt, the default setting was for people to get STEEM and SBD for their rewards. They would sell the SBD for STEEM and power up.

We still do this on HIVE.

The integration matches STEEM rewards with TRX rewards. The result is that everyone opts for 100% power up to get more TRX.

So the people selling SBD for STEEM disappeared.