Physics professor David Kagan examines the drop in the drag coefficient of the current baseball. His calculations of the number based on the probability of a well struck ball leaving the park agree with measurements by the lab that authenticates baseballs.

Now we are prepared to get back to the original question: Why is it so hard to figure out the cause of the drop in the drag coefficient?  Back in my garden, I can’t easily detect a decrease in the average size of a tomato because the tomatoes themselves vary in size much more than any change in the average size. In the lab with a bunch of baseballs, we need to explain a 3% change in drag coefficient when the drag coefficient naturally varies over a 40% range.  

If I was really concerned enough to find out if the fertilizer was worth the money, I would need to measure the size of very large numbers of randomly chosen tomatoes both before and after the fertilizer.  That way, I would be very certain of the average size in both cases. Similarly, if you suspect some property of the ball (seam height, surface roughness, etc.) has changed, you need to know the average value of that property both before and after to a high degree of certainty.

The point is, you can’t just grab a few baseballs and test to see if some particular property of the ball is the source of the change in drag coefficient.  After all, you may have randomly grabbed some that have too high a drag coefficient and get really screwy results. The underlying issue is that we’re looking for a needle in a haystack–and you need to examine a lot of hay before you find the needle.

THT.FanGraphs.com

Maybe the right thing to do now is stop asking why, and start asking what would bring the drag coefficient up again.

Note, too, that a lot more balls were well hit in 2019, so the increase in home runs isn’t all on the baseball.