Rumor has it it's been in the US since at least December. Maybe we could've started flattening the curve sooner, who knows? But if you think about it, people were traveling heavily for the holidays already. If this was spreading in China as early as November, how hard would it have been for a bunch of them to come to the US in say early December and kick off the flu season with a bang?
The holiday season is a major time for companies, too, many of whom do the bulk of their business around that time. For all we know, many prominent people knew a lot sooner but didn't want to cancel Christmas because of all the revenue they would've lost.