As if the other recently announced changes to Horizons weren't enough even more have been announced today:
The full article can be read here, and you can visit the official Horizons site here.
This limit of five also prevents much centralization, as is not really practical for a crafter to use a single town as a home base. To be effective, you really need to spread your items out among three or four communities in order to get an effective number of items listed. This is a significant frustration to some sellers.
After some debate, one of the upcoming changes will be an increase in the allowed number of items on a single consigner, from 5 to 10. This is a temporary increase while we discuss further changes, one of the proposals for which is outlined below.