I'm with you on this it is a bit odd. If I was Niamh Perry or Leanne Dobinson I'd be at the front of the queue for those auditions. The likes of Sarah and Rachel may not want to do the role again, and there maybe some who would never go through that process again (I'll bet Fran wouldn't).
The problem is they want a prime time pre-watershed show and they need a popular musical that appeals to all ages. That limits what they can use. I'd have loved them to put Singing In The Rain on the stage and look for a Don Lockwood, but that's just me. I can't think of any family musicals that have a male lead.
Actually personally I would put the show on after the watershed and look for a Frank-N-Furter in The Rocky Horror Show, or EmmCee in Cabaret.
Btw, which Maria played Dorothy?