Because it's very fitted over the waist/hips/butt. The pencil skirts on J Crew don't even look as fitted, and anyway they only emphasize one body part. A sleek, fitted dress shows all your curves. Look at Joan on Mad Men. Yes, technically her outfits look like office wear, but she also looks a frickin' bombshell. That's more attention than you want at work in modern times.Why do most of you think that suiting dresses are sexy? I'm thinking of dresses like this.
I don't think they are much sexier than a fitted pencil skirt with silky tank top. It just seems like business casual to me.
But again, depends on how you are built, how fitted the dress is, and whether other people wear something similar at work. If you are straight up and down, I think it can be a great menswear look. If you're Joan, then a sheath is probably more appropriate.
OP, you could also wear a fitted dress out and about during the day, and dress down with a cardi if need be.