I am not an epidemiologist (it took me three tries to even spell it correctly), but here's my baseless internet speculation. Maybe seasonal coronaviruses have this short-lived immunity because they've been been exposed to a lot of selective pressure for it. If our bodies produced long-term immunity to a coronavirus, it'd tend to die out, while ones that only induce short-term immunity would tend to survive by re-circulating. So maybe it's sort of obvious that common coronaviruses would have short-term immunity, but that doesn't really tell us much about a random novel coronavirus? Or is that crazy?