So the question before the jury has a significant component of "Should he have found out by this time?" Which is a question of fact, and facts are typically decided by juries, in the US at least.

The two parties can agree together to let a judge decide facts like this, but generally, if one or the other party wants it to go to a jury, it does.

I'm guessing part of Musk's strategy was to have it go to the jury, which are often seen as easier to manipulate than judges, especially when a case is weak. Or perhaps his team already knew this particular judge would be inclined to rule against him, so did the next best thing.