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
If I understand you correctly, you mean Bitcoin is stronger in terms of its market price. It is noteworthy that market cap of Gold significantly surpasses that of Bitcoin. Currently the market cap of Bitcoin stands over a trillion US dollars, whereas Gold market cap is estimated over 12 trillion. This stark difference is because traditionally Gold is perceived by vast population, as most secure and safe heaven investment since centuries. While Bitcoin is relatively new class of asset in the modern world, and its widespread adoption will take time to materialize.