Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They typically indicate location, direction, time, or manner. Examples: on, in, under, before, after, with, by.
* Adverbs modify verbs, adjectives, or other adverbs. They provide information about how, when, where, or to what extent something is done. "Greatly" tells us the degree or extent of something.
Example:
* Incorrect: I walked greatly the park. (This doesn't make sense because "greatly" doesn't show a relationship between "I" and "the park.")
* Correct: I walked greatly through the park. ("Greatly" modifies the verb "walked" and tells us how I walked.)