The problem with using old synonyms is species are often renamed because they were considered to be apart of a described species when in reality they were a unique species. Alternatively, two populations might be called two different species until evidence suggests they actually belong to the same species. Finally, species are routinely moved from one taxon to another in order to remedy paraphyly. In all three scenarios, using an outdated name would misidentify the species and thus be avoided.
In biology, it is important to give credit to the original author of a species, and so species names are not changed unless they’re incorrect.