Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

Why do volatility stocks/ETFs (TVIX, VXX, UVXY) trend down in the long-term?
Since these indices only try to follow VIX and don't have the underlying constituents (as the constituents don't really exist in most meaningful senses) they will always deviate from the exact numbers but should follow the general pattern. You're right, however, in stating that the graphs that you have presented are substantially different and look like the indices other than VIX are always decreasing. The problem with this analysis is that the basis of your graphs is different; they all start at different dates... We can fix this by putting them all on the same graph:  this shows that the funds did broadly follow VIX over the period (5 years) and this also encompasses a time when some of the funds started. The funds do decline faster than VIX from the beginning of 2012 onward and I had a theory for why so I grabbed a graph for that period. My theory was that, since volatility had fallen massively after the throes of the financial crisis there was less money to be made from betting on (investing in?) volatility and so the assets invested in the funds had fallen making them smaller in comparison to their 2011-2012 basis.  Here we see that the funds are again closely following VIX until the beginning of 2016 where they again diverged lower as volatility fell, probably again as a result of withdrawals of capital as VIX returns fell. A tighter graph may show this again as the gap seems to be narrowing as people look to bet on volatility due to recent events. So... if the funds are basically following VIX, why has VIX been falling consistently over this time? Increased certainty in the markets and a return to growth (or at least lower negative growth) in most economies, particularly western economies where the majority of market investment occurs, and a reduction in the risk of European countries defaulting, particularly Portugal, Ireland, Greece, and Spain; the "PIGS" countries has resulted in lower volatility and a return to normal(ish) market conditions. In summary the funds are basically following VIX but their values are based on their underlying capital. This underlying capital has been falling as returns on volatility have been falling resulting in their diverging from VIX whilst broadly following it on the new basis.