Because it's still new and in beta. Their's no guarantee yet that Steemit will be a Facebook killer.
But every day the platform survives, more unique content get's created and so the overall value of the block-chain should increase.
So a stable price of Steem means that the underlying value is growing faster than the coin. Which means that the coin is undervalued.
@jcornel hey your post came true! steem went to fucking $8 + in jan 2018 lol if only we had perfect hindsight