I agree with you historically, but part of the blame goes to the Roman Catholic Church which teaches (at least in traditional circles) that Constantine was "Catholic." You can listen and watch traditional Catholic sermons on You Tube and hear exactly that!
The West was exposed to the RCC and the Reformation was expressly a reaction to Vatican's autocratic tendencies, just as the Schism came out of a power struggle for universal jurisdiction of the popes. We are to a great extent, to this day, unwilling to share our Eucharist with the Latins and their followers from other rites precisely because of that same reason.
We generally consider any early church father Catholic, as well as, of course, Orthodox. The precedent was set for St. Ignatius in the Letter to the Smyrnaeans.
In the case of Emperor Constantine it is problematic because he was not baptized till his deathbed. Is it why you consider it improper to call him Catholic?