Our author, Thomas More, is also our first person narrator. Yep. How's that for confusing?
This wasn't all that uncommon at the time. In fact, since discovery of the new world was an actual historical reality (check out "New World Discovery" in Symbols, Imagery. and Allegory) real-life explorers, like Christopher Columbus, were writing accounts of their discoveries in the first person. More was just channeling those accounts.
It works, too, because the whole thing contributes to the "I swear this actually happened!" We really feel like we're listening to an actual account:
"It would take too long to repeat all that Raphael told us he had observed in various places, nor would it altogether serve our present purpose. Perhaps on another occasion we shall tell more about the things that are most profitable" (1.12)
There More goes, trying to give the effect of actually being there and having to deal with the constraints of time and memory.
Oh, one other thing. Remember that Utopia's first person narration is a first person account of another first person account. While More suggests that they might chat with this (non-existent) person again in the quotation above, there's something a little fishy going on with how well he recalls Hythloday's very long and detailed description of Utopia. Just saying.