CoinDesk reported:
From the very beginning, observers speculated BITO and other futures-based ETFs would significantly underperform bitcoin due to costs associated with rolling over, or selling expiring futures contracts and buying the next set. Usually, longer-dated futures contracts trade at a premium to those closer to expiry, a condition known as contango. The contango tends to steepen during bull runs, and the steeper the contango, the higher the costs, and the so-called contango bleed.