My guess is they wanted someone on the larger side for the role. Geralt is a pretty big dude and very athletic. Cavill is over 6 foot, just under 200 pounds and in peak shape, so I can see him making a pretty intimidating witcher if they get the look right. Michael Wincott would have made a great Geralt, from the general look and voice, but they'd have to work some movie magic because he's not that tall and has a pretty slim build. If you've never seen his role in The Crow, just search YouTube with his name and "the crow" and you'll see what I mean.