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!

From a linguistic point of view, the missing article is called a Zero Article.

Several sections in the paper Determinants of Zero Article Use with Abstract Nouns: a Corpus-based Study are relevant. It starts off:

The zero article is normal with a number of non-postmodified abstract nouns even in cases where these are not used generically...

It proceeds to give examples. While "turnout" is not one of them, the following descriptions fits eerily well anyways:

Substituting the for zero in front of these nouns normally yields equally acceptable sentences, and would indeed be the normal choice in conversation, as transcriptions of speech and native speaker elicitation suggest...

The paper then proposes this conclusion as to why:

It would be more reasonable to view this kind of zero usage as a text-type-specific phenomenon limited to a distinct set of nouns and motivated by the desire for economy of expression.

Of course, this isn't a good explanation for your student (or anyone besides linguists). You should probably just tell them that while the original sentence is idiomatic, it's also idiomatic to use the there. But English is inconsistent; thinking too much about definite article rules "will send you to the hospital" in American English, but, in British English, it "will send you to hospital" ;)