They've seen enough of Castillo to think he's not worth a roster spot. Can't say I disagree. Maybe he sucks in practice too and it's not just what we saw on the field?
Young is a fine professional player. He's been in a slump so far, but if he replicated his 2015 season, we'd have no reason to complain. He might not have Castillo's "upside" but his floor isn't as low as Castillo's either.