Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. "Prope" describes the location or proximity of something, which is a function of an adverb.
* Adverbs answer questions like "where?", "when?", "how?", or "to what extent?". In the case of "prope," it answers the question "where?".
For example:
* Prope flumen habitabat. (He lived near the river.) Here, "prope" modifies the verb "habitabat" (lived) by specifying the location.
While "prope" can sometimes function as a preposition, its primary use in Latin is as an adverb.