Because Bitcoin was created in 2008 whereas gold has been a store of value for thousands of years.
...although gold has been into existence before creation or discovery of bitcoin, and them gold has more values.
And that length of time that gold existed can also prove how it is also unstable. Bitcoin is still a newborn comparing to the years gold is used but even though with its instability, in the long run, it is also giving stable profits to people that is why more and more people are drawn to it. Gold may be stable in the long run too but bitcoin can give more profit from it but with a higher risk.