Because the point wouldn't be to care if it was authenticated eventually. They are using the info for political purposes. They don't really care if anything like criminal charges come out of it. If they cared about that, then it would have come out sooner, since we know that they did have this info at least weeks ago, if not months. So long as no one can authenticate or prove them wrong within the next couple weeks, then they have placed doubt in some about voting for Biden.
There is info that Burisma was hacked last year though. That they did experience a data breach.
Russian military hackers tried to steal emails from the Ukrainian energy firm where Hunter Biden, the son of Democratic U.S. presidential contender Joe Biden, had a seat on the board, an American cybersecurity firm said on Monday.
www.reuters.com
Note in the story above that the timelines mentioned coincidentally match right around the two mentioned dates within the source story we have for this, Mar/April 2019 and Nov/Dec 2019. But this story came out before anyone had those particular months as references.