Wonder takes an embedded interrogative complement with its own internal trace:
You wonder who John saw t.
You wonder who t saw John.
You wonder why I left t.
When you front that wh- you're asking it to do double duty, in both an external interrogative and the embedded interrogative: in effect it's standing for itself rather than for the trace!
It's the interrogative role of the wh- complement which prohibits this movement. Note that this pair sustains the movement, because the original wh- clause is deployed not as an interrogative but as a pronominal:
I know who saw John. ⇒ okWho do you know saw John?
But this pair doesn't, because the original wh- clause is interrogative:
I'd like to know who saw John. ⇏ ∗Who would you like to know saw John?
Some grammarians distinguish these two uses of the wh- clause as 'embedded questions' and 'free' or 'fused relatives'. I don't, because the internal structure of the wh- clause is identical; but the difference in the external syntactic roles justifies a distinction at that level.