Is there a reason why 'agus' isn't used here?
Because of how Irish combines predicative adjectives, you don't need it. It's just a quirk of how you'd translate it.
Saw this question in another thread: how do you know that 'deas' isn't modifying 'scéal', as in 'the nice story is simple'?
I can't think of a context that would make it clear in this case.
You can tell from the way it's said 'SCÉAL deas' simplí versus scéal 'DEAS simplí'
Does it matter whether you say deas simplí nó simplí deas?