The original actor who played Maddox, Brian Brophy, doesn't act any more, he's now the head of the theater department at CalTech. So I'm thinking he didn't come back because he's just done with acting and didn't want to, or just couldn't find time in his schedule to do it.
As for Icheb, that one is a bit more of a surprise since the original actor did reprise the role in a fan film a couple years back, so he's obviously still willing to do it. I did see a something about some nasty comments he made in relation to Anthony Rapp, so that could possibly have something do with it. There are a lot of the same people working Picard who work on Discovery, so they might not have wanted to have anything to do with him after that.
Thanks for the info. I saw that fan film I think. Was it called Star Trek Renegades?