Because Tim Minchin et al looked at it and thought "well, this is obviously a man's role." (true story)
The RSC always has a blind casting approach.
The role is incredibly physically demanding (not saying a woman can't do it) but it involves lifting children up and stuff like that. The character also needs to tower over Miss Honey, and it's hard to find women over 6ft, sadly.
Also - Dennis Kelly saw it that the character is neither a man nor woman... it's a monster.
I've heard rumours that Bertie will go over to do it - as he's not doing Bridget Jones' musical any more....
The RSC won't cast famous names in it - so don't expect Sutton Foster or Harvey Fierstein. Roald Dahl is the star of this show - and that's how it will remain.
Nick Hutson
Co-Presenter/Producer
MusicalTalk - The UK's Musical Theatre Podcast
http://www.musicaltalk.co.uk