Well...true. As much as I would have loved Cavill popping up in part 1, it wouldn't have made sense like Wonder Woman popping up in part 2 would.
Plus looking at the timeline, Shazam 1 came out roughly two years after Justice League (Whedon cut), a film that WB wanted to be the end of Cavill's Superman anyway. They still see life in Wonder Woman, and it would be inbetween her third (and final?) film. 5 appearances total vs Cavill's three, six if she cameos in Shazam 2.
That's why I'm hoping whatever sequel they come up with for Black Adam, Cavill's Superman gets a cameo, even if its a sort of "goodbye and thanks, Henry Cavill" type of cameo, similar to what it appears Affleck's Batman is getting in "The Flash".