Here's why:
* "Frankly" as an adverb: When "frankly" modifies a verb or an adjective, it functions as an adverb, and adverbs don't typically need commas before them.
* Example: "Frankly, I don't understand why you're so upset." (Incorrect - comma not needed)
* "Frankly" as an interjection: If "frankly" stands alone as an interjection expressing a strong opinion, you can put a comma after it.
* Example: "Frankly, I'm surprised you didn't say anything." (Correct - comma used because it's an interjection)
* "Frankly" introducing a parenthetical element: If "frankly" introduces a parenthetical phrase or clause, you can use commas to set it off.
* Example: "I think, frankly, that we should reconsider our options." (Correct - commas used because it introduces a parenthetical phrase)
In most cases, "frankly" acts as an adverb, so you wouldn't use a comma after it.