grammaticality - Why is there no definite article in this sentence?

So today my Japanese student asked me why there is no "the" before "turnout" in the following paragraph.

But Becker said that while turnout in purple states like Florida and Pennsylvania had a slight uptick this year, at least 19 other states saw lower turnout rates compared with 2012, a scenario that is antithetical to presidential-year voting that tends to increase each cycle when an incumbent is not a part of the race.

(Source: What does voter turnout tell us about the 2016 election?, PBS Newshour, 11/20/16.)

I ended up saying that we can add "the", but it's not necessary, so it's a case where you can add "the" or exclude it. My student became confused, and I feel like I provided a bad explanation. ( or a wrong one at that )

Can someone please help me explain this to him?

Thank you to whoever is willing to help out!