Cost and accuracy.
And very importantly ... physical size.
Although it's possible to have certain capacitors of similar size with differing voltage ratings, capacitors vary widely in construction. Different construction results in differing sizes, and in some cases, a certain construction will result in large variances in size from one voltage rating to another. The size of modern capacitors is smaller, generally, than older variants. So, it is easy usually to substitute a larger voltage rating unit when doing such things as repairing collectable radios. If you are building new, however, size is an important constraint. For example, short signal paths in a Switching Amplifier (eg Class D) are important, as are capacitors mounted very close to the chip amp. Size will be critical here.