Quite likely because the only dudes who knew Latin in the Middle Ages were in monasteries, and so they applied Latin words they learned to their circumstances. "Hey, it says here that when Romans brought in a new imported, inexperienced slave, they called them novicus. Let's use that for our newcomers! After all, we make them do chores, like slaves, LOLZ!"

https://www.etymonline.com/word/novice

Sure the root word might be Latin, (and so is a majority of European vocabulary), but the specific usage of "novice" meaning "newbie" in English, comes from the monastic orders, via French.

Monastic orders, which, in fact, did not "do stuff, like slaves, LOLZ", but were instrumental in preserving (or rediscovering) knowledge and technology from the imperial era.