The only people who are buying futures are those who are at the same time manipulating the normal market. If you wanted to invest why would you buy futures instead of the real thing? But if you wanted to sell a lot of bitcoins or knew someone who is going to sell, suddenly futures start looking like a much better option.
These futures appeal to institutions because they don't care about the real thing, they are only out to ride its exposure. Why would they buy Bitcoin when this is just as efficient?
They don't want the hassle of storing coins, which isn't as easy as it may seem. People think a paper wallet or hardware wallet is enough, but it's not. Securely storing millions worth of coins goes way beyond that.
If you look at gold for example, most of the volume generated consists of derivatives tied to it, and not the real thing itself, and the same will happen to Bitcoin when more and more institutions enter the market.