There is no underlying limitation, but rather that in most cases, a filter only makes sense if you want to update (or delete or whatever) a subset of linked items, therefore it is meaningless for to-one relations.
It would be solved if/when there was the option to define trigger filters using formulas, in which case, you would only trigger the automation in the case where the related entity meets the required criteria.
In your case, the current workaround is a lookup / formula field on the triggering entity.