Why are some pitot tubes more prone to icing than others?
Actually the answer is quite complex, but essentially depends on the local aerodynamics, shape of the probe and the anti-icing power (heating).

Imagine an airplane flying around a cloud, it essentially approaches air with small drops of water floating around, those drops of water impact the pitot tube and some of them keep over the pitot tube surface.

Now, the key question is if that water will or will not become ice. That essentially depends how long the water keeps over the surface and the local temperature.

Keeping on the surface depends on the shape of probe and the local pressure. The water temperature will depend on a balance between the external temperature (also dependent on the local pressure) and the balance of power applied to the probe to avoid freezing.

So, the phenomena is quite complex and depends on all of these factors, and the potential case of pitot tubes prone to icing will likely being a condition unexpected and outside margins as all these factors are taken usually into account.