I can only infer the reason why Bitcoin's investment is preferred than oil, because Bitcoin is commonly used by everyone so anyone can invest in it, in other words people who can use their Internet can easily get involved in investing in Bitcoin. While oil investments can only be felt by businessmen and everyone can not easily invest in it.
Literally anyone can invest in oil directly by buying oil contracts. It's not any more difficult than buying bitcoin, you just need a brokerage account. You can also invest in oil indirectly by buying stock in any company that produces oil. And far more people use oil products than bitcoin, it's not even close.