Why does the trochlear nerve arise dorsally and cross over?

[Xalatan]

When other brainstem cranial nerves don't?

[imatfaal]

To allow examiners to set more interesting questions about ipsilateral and contralateral indications of lesions? Not sure there really is a why

[Function]

No idea why it does specifically exit the brain stem dorsally, but a small correction (very small): your title gives the impression that it decussates after leaving the brain stem. Note that its fibres already decussate in the brain stem