Mostly because STEEM is an inflationary currency.
Since there is a proportion who will always sell their new STEEM, then price is equivalent to the that proportion divided by the incoming money.
Or, there is a US$ reward pool that actually gets drained each month.
Most of the alt-coins being compared to have an equal income as outgo.
Also the only compelling reason to hold on to old STEEM is to influence the output of new STEEM. There isn't a compelling reason to hold versus sell your gains. Most people will tend secure the profit. They aren't selling their Steem Power, but they are selling the inflation which consistently places downward pressure on the token.
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