There is also an immense amount of water vapour being produced by the combustion of a hydrocarbon.

Sure, but water vapor doesn't spontaneously transition to a liquid and accrete onto surfaces - there needs to be a super-saturation of water vapor, and given the temperatures of jet exhaust, that's not trivial to achieve. However, the super-saturation needed for water vapor to deposit onto surfaces as ice is much lower, hence the preference for ice crystal nucleation.