BOIL is designed to be used for short term trading and only truly tracks natural gas on a intraday basis.
BOIL decays because it is a commodity ETF and leveraged ETF.
Commodity ETFs have to roll futures contracts, natural gas is typically in contango causing BOIL to decay.
Leveraged ETFs re-balance each day to maintain the leverage, re-balancing results in volatility decay.
BOIL can be used for short term trading but as you can see by this chart should not be held for long periods of time.
Do Not Buy and Hold