The value at coinmarketcap.com are denominated at base in BTC so the value of BTC has an influence on the figure as well. If BTC goes down, so does this number. A more complicated tracking system would be required to see that value in dollars, and besides, Steem can be traded for several other tokens as well, and is, so really, that value should account for multiple pair crosses... The numbers of different tokens going into exchanges is getting dizzying.