The Song of the Year award goes to the songwriter(s) and as opposed to a catchy single, ostensibly the song needs to stand on its own. In truth, it's difficult to separate a song from the recording of the song. Voters are human, not AI. It's doubtful that any CMA voter sits down with a yellow highlighter and scours a nominated song's lyrics. Thus, it's common for the same song to garner both single and song of the year. 
But let's pretend the winning song actually wins on merit.  I've already listened to four of the five nominated songs, since they are also Single of the Year nominees, but this time around I'll try to disregard the performances and focus in on the songs themselves.
Here we go....
Fast Car, songwriter Tracy Chapman
This video benefits from having scrolling lyrics, since the song structure tends to make the words run together. Nevertheless, this is a fine piece of writing. It's written in a stream of consciousness style, the woman (in Tracy's version) matter-of-factly laying out her present circumstances and her dream of flying away, which is where the chorus soars. Then she talks about the disappointment her new life has become. The end chorus repeats her (now fading) dream, but signals that she hasn't yet given up hope. From a technical standpoint, the rhyming isn't there, but it hardly matters. This is a song that says something.

Heart Like A Truck, songwriters Trannie Anderson, Dallas Wilson, Lainey Wilson

The chorus is well-written. It seems like the verses were added as an after-thought. They should have been tightened up. Ending every line with "yea" is a weak substitute for finding rhymes that actually say something. The performance video led me to believe that the singer was another "powerful woman", which is an irksome sentiment. A powerful woman doesn't need to announce that to the world. However, the line, "it's as good as it is tough" tends to soften it a bit. This song was a good initial idea that didn't live up to its promise.

Next Thing You Know, songwriters Jordan Davis, Greylan James, Chase McGill, Josh Osborne

The performance of this song outshines the writing. (Really, four people?) Like Fast Car, it, too, is written in a stream of consciousness style, and it's...fine. It hits all the high points of a life. But it's rather mundane. I wonder how the four songwriters split the song ~ one wrote the bulk of it and the other three changed a word here and there? I'm rather disappointed in the song itself, since the performance video really grabbed me.

Tennessee Orange, songwriters David Fanning, Paul Jenkins, Megan Moroney, Ben Williams

This is the only song of the four I hadn't heard before, and okay, it's kind of dumb. As if no one has ever met someone from another place before, and oh, he roots for the wrong team. From a songwriting standpoint, this is technically fine. A three-quarter-time song is the easiest song to write. I'm shocked it actually required four songwriters. I'm equally shocked that this ended up on the short list for Song of the Year. Song of the Year should stand the test of time. This is a throwaway.

wait in the truck, songwriters Renee Blair, Michael Hardy, Hunter Phelps, Jordan Schmidt

(sigh) This song makes me feel sad for the state of songwriting. Or maybe four of the five song nominees make me sad. It's like people forgot how to write songs. Or that they try too hard. I was more surprised by Carroll County Accident than I was by this. And frankly, the protagonist is a dumb ass. And that choral ending really dilutes any power the song possessed. Did all four songwriters simply give up? No wonder the CMA is digging through the nineties catalog to find well-written songs.
My clear choice for Song of the Year ~ Fast Car ~ writer Tracy Chapman.
My scorecard (and this is really difficult):
  1. Fast Car
  2. Heart Like A Truck
  3. Next Thing You Know
  4. wait in the truck
  5. Tennessee Orange
Guys, try harder. Spend some time listening to the masters. And disband your four-writer clique.  
My God.