I mentioned this 10 years ago on this board when people wanted Sinise as McCoy for the 1st Abrams film, and I think it applies in this thread, too...
That is, I have always found Sinise to be lacking warmth in his acting. While I think he is a relatively good actor, I have a tough time emotionally connecting with him and his characters -- and I think it is due to that (IMO) lack of warmth; i.e., Sinise's portrayals have almost always seemed cold to me (with the exception of Lt. Dan).
Granted, Karl Urban is not particularly "warm and fuzzy", but I have often been able to emotionally connect more with Urban's acting portrayals than with Sinise's. DeForest Kelley's portrayal of McCoy made him out to be irascible, but with a butt-load of humanity mixed in. I think Urban captured that pretty well, possibly better than Sinise could have captured it -- especially the "humanity" part.