To annoy him.
I hold that Kal-El is all one name and it shouldn't be shortened as if it's an English name. If Diana wants to pay respect to his Kryptonian heritage, then call him Kal-El. If she wants to be monosyllabic, then call him Supe.
This is just my head canon, given so many writers make it just Kal in hundreds of stories. But I prefer Kryptonians and their names to be culturally distinct.
The "Kal" thing bugs me about as much as "Polly" for Hippolyta. Superman should use "Na" for Diana and see how she likes it.
"Superman" is impersonal, and calling him "Clark" in public is off-limits.
Becauae its more of a turn on for Clark when she screams his real name out when they serve "Truth and Justice" with each other, and she knows it.
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“Look, you can’t put the Superman #77s with the #200s. They haven’t even discovered Red Kryptonite yet. And you can’t put the #98s with the #300s, Lori Lemaris hasn’t even been introduced.” — Sam
“Where the hell are you from? Krypton?” — Edgar Frog
I think out of the common Justice Leaguers. Only Bruce and Diana call Superman "Clark" because they interact with his human identity the most.
I think Timm Universe called him Superman until the Hawk invasion where they all knew each other's identities.
Only New52 and DCAU WW calls him Kal-El regularly. I think Aquaman, Thanagar Hawkman and Martian Manhunter call Superman by Kal-El too.
"Cable was right!"
I mean, the real answer is that it depends on who's writing at the time. But how do we think she should refer to him? It's got to be Clark. She's the goddess of truth, and at his heart he is Clark Kent. Yes, his birth parents called him Kal-el, but then they gave him up (whether they wanted to or not, no value judgement on it). If you're adopted at birth and your adopted parents give you a name, birth parents can't come in and say "Man, you called him George? We really thought he'd be a Steve, let's go with Steve."
His name is Clark and a well written WW would call him that. If they're in public and its in a Clark-still-has-a-secret-identity timeline, then I can see her using Kal. I could also see her using it as a pet name/nickname for him in a cutesy kind of way. (HEADCANON: Clark spent 1 month as a moody teenager shortly after finding out about his kryptonian heritage and told his parents he'd only respond to Kal-el when at home. He tried to be all alien and aloof and it lasted about as long as it took someone to need him to make them smile. He told Diana this so she calls him Kal as gentle ribbing.)
I'm going with the interpretation that Diana uses Kal because they're both immigrants while Bruce calls him Clark because they're both raised Americans.