I didn't get into all the reasons, but Josh Stein of MMA Opinion raises an important one: Almost all of Silva's fighting experience is in Pride, which uses a traditional ring much like a boxing ring. UFC, of course, uses the Octagon, its trademarked version of the cage that is much more popular in American mixed martial arts.
Silva comes from a muay thai background and trained most of his career to fight in rings, and there are certain movement patterns that fighters use to escape off of the ropes that don't apply in the Octagon. The shape of the cage makes the geometry a little bit trickier and requires opponents to move a little bit closer to the cage, as more of a back-away and sideways motion, instead of the more traditional circling motion that boxers and thai fighters use to get off of the ropes. Wanderlei was, and still seems to be, attached to attacking fighters with that movement pattern.Jardine is much more experienced than Silva at fighting in the cage, and that's a real advantage. Yes, MMA is MMA, and the same basic skills are needed regardless of venue. But asking a fighter who spent his entire career in the ring to adjust to the cage is asking a lot. And in Silva's case, it may be asking too much.