These are three factors. Fields of kale are more open to wind for example.
And stress on plants leads to aphids – Steph says her kale in a walled garden is badly affected, and its worst under a tree where moisture has been short (=stress). But her allotment kale is fine, also helped by the compost mulch on top.
At Homeacres I have made my soil really fertile and have seen very few aphids.
If you have some spare compost, I would mulch your kale plants. Also I do think that Cavalo Nero is susceptible.