The toothbrush is meant to remove a growing living substance, namely bacteria. The shotgun cleaner is meant to removal particles.

After a period of time the toothbrush too will become a colony of bacteria. It will then add to the existing problem or at least offer no benefit. This is why you should change them so often.

The shotgun cleaner is not used to remove bacteria primarily and will not suffer the same issue provided the particles/grime can be removed from it.