Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* "Alligators" is a noun, the subject of the sentence. It names the thing being described.
Let's break down the other words:
* Unfortunately is an adverb modifying the entire sentence.
* For is a preposition.
* Us is a pronoun.
* Can is a helping verb.
* Run is the main verb.
* Short is an adjective modifying "distances."
* Distances is a noun.
* More is an adverb modifying "swiftly."
* Swiftly is an adverb modifying the verb "run."
* Than is a conjunction.
* People is a noun.