They're probably not the main reason they're picked, but complicated mechanics seems to be a pretty consistent marketing point for the DLC as of current.
Heck, the two characters we've got in vol 2 are probably the most unSmash-like characters we've ever gotten
I get that, But that doesn't automatically mean every Complicated mechanic will get in
What I mean is that every Complicated mechanic has it's own issues and such, Some more than others. Now We don't know which Mechanics are too intensive for FP2 as we're not the ones Developing the game, and that's why I say Rex and Dante, Even though I think they won't be playable in FP2 and both are very complicated, they still are likely characters
But There's a Balance though with the Character and Moveset. We know that Characters like Heihachi, Mach Rider, and Chorus Kids were considered, but dropped, and I bet a major factor in all of these was complications with movesets, So Not every Complicated mechanic is easy to Transition as another.
and it depends on the character too. the Character has to be worthwhile of an Option to drop development time on them, like if Mach Rider was Nintendo's Best selling I.P, I think Sakurai could find a way to make it work perhaps, but considering he's irrelevant it's an harder Sell. Steve and Hero are massivly popular, so despite how Intensive their development was, they were worthwhile. It's also something to note that Steve was easy to Animate, had his Model Ported from Minecraft, and sound design was easy. Hero re-used assets from Other Sword Characters like Link
So While I still think Rex and Dante have shots, There are other Characters to be considered as well, some might have easier to Transition Movesets, along with being more popular, requested, or benefitting Nintendo more than these 2. That's what I Mean