Because post is more than just letters, it's parcels too. Canada Post is infrastructure that ties the whole country together, not just the denser, more profitable cities. Imagine if there were only for-profit postal services in the country. What would it cost to send a parcel to 100 Mile House, or Baker Lake, or whole swathes of the country that only speak French? Think of all the things that go out by post, like Carbon tax rebate cheques and voting information. It'd introduce a massive disparity in service and access to basic services, and so we socialise that cost across the country.

There are always ways to improve of course, but you asked specifically about why the system was socialised.