The biggest reason is that it looks like this should have been a two-part model rather than fusing together the stone under the beast's body. It just looks strange to me. Given the dynamic and multi-part sculpts some of the other models in this game have, it is clear they are capable of more on this one.
Secondly, the teeth and face in general look much more 'human' to me than the more ape-like representation of the game art.
To add to my initial negative opinion of the model itself, I painted this one when I was already a bit tired. Eventually I just sort of gave up adding highlights and additional detail and called it done.
Overall I don't think it turned out 'bad', just less than inspiring and terrifying for a level 3 monster.