I'll expand a little more. This is a privacy issue as well. By adding a polling division on a special ballot, a person's identity could be ascertained. Given the relatively small number of people who would vote by special ballots, having the polling number could potentially identify how an individual elector voted in a lot of cases. This is a privacy issue. It's to ensure their votes are anonymous, as they ought to be.
On October 16th, 2018. See this statement in context.