I think because of Henry Cavill's size (not to harp on it and NOT an issue with me) however I think they need to find someone that has somewhat of a similar size.
With that said, my top choice based on acting chops, and that sort of good guy/bad guy and dark look similar to Bale,
Sam Worthington 36 years old will be 37 next month 5'10
James Badge Dale 35 years old 5'11
If age is not a factor, then my third choice would be
Andrew Lincoln 39 years old 5'10
All of these actors are solid. Chris Pine could be another suggestion, Cillian Murphy. If I had to choose, I would base it on star power, age, look, acting -- I would go with Sam Worthington. James Badge Dale could end up exactly like Christian Bale (not very well known yet but intrigues the audience.) so he would be a solid choice too. I hope they go with someone that can act and not a complete unknown.