Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* Currently tells us about the time of an action or state of being.
For example:
* Currently, I am writing this response. (modifies the verb "am writing")
* The weather is currently quite pleasant. (modifies the adjective "pleasant")
* He is walking currently very slowly. (modifies the adverb "slowly")