I think there are two ways of looking at it, and that they are ultimately equivalent.
As others have said, it's an insurance premium, as simple as that.
There is only one thing which affects the price of VIX futures: supply and demand. You can choose to be the insurer (sell futures), in which case you earn long-term profit from premiums, but take huge losses from time to time. Or choose to be the insured (buy futures) and make long term losses (the insurance premium) in return for protection from unpleasant volatility.
No sane insurer would offer their services if there wasn't a substantial reward for their taking on risk. No reasonable insuree expects to get insurance for free. Nothing but supply and demand cause the value of a rolling contract to decay according to the inferred value of the insurance premium.
The other (ultimately equivalent) way to look at it is this: one can approximately replicate the risk profile of a long VIX position by buying strips of call and put options. Or one (obviously) can roughly replicate the risk profile of a short VIX position by writing strips of calls and puts.
Option pricing (for the same underlying reason... that buying options is like buying insurance and writing them is like selling insurance) is such that in the long term, the option writer (like the Casino) wins. A strategy of continually buying strips of puts ultimately loses money exactly like VIX future decay, and a strategy of continually writing strips of puts ultimately appreciates exactly like a short VIX position.
Thus the presence of an efficient options market (and the existence of arbitrage) determines that VIX futures must exhibit the same behaviour as equivalent options strategies, i.e. decay for the buyer, long term profit for the seller.
Oh, and don't for a minute believe that either options or VIX futures are priced by any mechanism but supply and demand. Black-Scholes and derived models may inform the options market, but at the end of the day it is just a market, priced according to supply and demand. Similarly, the published VIX index informs the buyers and sellers of VIX futures, but supply and demand dictate the price.
And supply and demand, since we are naturally risk averse, ensures that ultimately insurers make long term profits, and insurees make long term losses.