It may not be the mainstream investment right now and no doubt gold is still preferred by many investors. It is because they still don't have an idea of what bitcoin is. For me they are good investments depending on where you want to depend, but having them both is better.
Why having both is better? I think everyone should decide for themselves what is better, either gold or Bitcoin, and invest in that one thing. For example if you know that Bitcoin is better why invest in gold? Yes it's more risky to invest in one thing but on the other hand you'll have greater profit in case you were right.
IMO people heavily overestimate gold and in the future the price of it will be falling. It will never be zero though because gold is a precious metal, but, like I said, heavily overestimated.
Are you asking why having both is better?for real? If you are an investor and you know both things are profitable. Why put all your eggs at one basket? What makes you think that gold's value will be falling in the future? You are underestimating what gold is. It's one of the most stable investment in the world and it is still precious.