There are two reasons. First, Bitcoin is a non-credit currency ten times stronger than gold. So unless Bitcoin is replaced. Otherwise he must surpass gold. Second, Bitcoin is close to being the ideal non-credit currency, and it is basically impossible to have another non-credit currency beyond Bitcoin. Combine these two points and Bitcoin is bound to surpass gold
As I see it, there is no point in comparing Bitcoin with Gold. Both are completely different things. Sure they have been both used as currency but that is really the only relevant thing they have in common.
Bitcoin surpassing [insert valuable thing here] is only psychologically pleasing to those for whom gold was not already worth less than whatever. But for the people who already thought that Bitcoin is worth more than gold, it will not make a difference.
Bitcoin is unmatched and unique. Something which did not exist in the past. A new generation of money. And if you compare it to gold, it makes it seem worth less, in my opinion.