Because the pelvis itself juts out quite a bit, plus there wouldn't be that much fat on them, plus no lizard/reptile have their legs directly under their body, which shifts their body style. For birds they actually do stick out, but it is just covered by feathers.
But yeah, nice work on the Acrocanthosaurus. (Now if only I had $650 for Zbrush.)
Well as wonderful as Zbrush is there is a free alternative to anything. I was fortunate enough to be gifted a copy of Zbrush, but I sculpted digitally long before I actually had it. Its always good to get the feel of something and be sure you are into it before you throw a bunch of money at it right?
I'd suggest you try out sculptris, if you haven't already, which is a great little free program by the same guys who made Zbrush. As well Blender has started to come out with some interesting sculpting tools that I've seen some nice stuff done with. So check either of those out if your interested in digital sculpting for sure. Your not going to get quite the same robust tool-set that Zbrush or Mudbox has to offer, but then again that's why they cost the big bucks.