Your premise is severely flawed.

First, it assumes that forks are old. They are not.

Second, it assumes they were once in use in China and then stopped being used. This is not true, as they were never in widespread use.

Forks are a quite recent and for the most part were not the commonplace utensil they are today in the West until about the 17th century, and not really widespread in European cultures until the beginning of the 18th century. Prior to this, you would only find them in common usage in Italy.

They eventually became commonplace in Europe and by extension the Americas, but they are not some primordial utensil of eating. They didn't disappear in China because they were never a thing in China. They weren't much of a thing much of anywhere until the 17th century, when they were adopted.

Forks didn't fall out of use in China. Forks fell into use in Europe, and quite recently at that.