First of all, no one knows for sure. But here are a few reasons at play:
Supply: Roughly every 10 mins, 25 new BTC are issued ("block reward"). At ~$230 per BTC, that's ~$830K new BTC a day (25 x 6 x 24 x 230). As many miners sell most of the block reward to recoup the cost of mining, for bitcoin's price to remain constant, these new BTC have to be absorbed (=bought) by market participants.