I know this is an old topic, however I was recommended to read topic, so I answer this question

There are many institutional investors which cannot buy bitcoin directly, due to regulations and law.

For example, pension funds cannot buy bitcoin, the law clearly state what they can or cannot buy, after all they are investing someone else's money. There are other institutional investors in the same situation.
They can not buy a bitcoin, but they can buy a future, or an ETF.