Italy in the XV century - after the peace treaty of Lodi in 1454 - was composed of 7 big + give or take other 10 small states at least: it was quite fragmented.
Venice was having, in the Renaissance period, a big economical/cultural/political influence on the italic peninsula, and its "old" Carnival tradition - besides having given the seed to other Carnivals to burgeon - was so magnificent and flamboyant, that attracted people from other countries.
And every year, starting from december 26, other people from other states who wanted to take advantage of the concentration situation (i.e. Charlatans, Commedia dell'Arte companies, Fortune Tellers) and also regular people who just wanted to appear & have a good time would show up. Tourism? I'd say.
Different non-venetian-style masked individuals would be then seen in the St Marks' Square, bringing in money, new ideas and some more folly, which is never enough, coming to think of it.
And many of the masks from the outside also took inspiration from the Commedia dell'Arte, that was fast becoming an itinerant means of creating entertainment, of making the regular folks get a good laugh.
