I think Swiss Chard is a bit too limp, like spinach. I believe the kale likely stands up under the assault of roasting-level heat because it's a pretty tough, stiff vegetable, with a whole network of veins going through the leaves & those starched-up Elizabethan ruffs on the ends. The structure is just a lot stronger & so it doesn't wilt & curl in on itself.
[Have a tray in the oven right now. And also one of green beans.]