Yup, lack of environmental pollution is likely to be your
problem, aided by too much rain!
While I was looking after my daughter's late 2009
Ibiza SC, that sort of thing annoyed me, it was mainly on the side door window seals - again for me not enough pollution and too much rainfall.
I'd be very wary about using a "soft toothbrush" as that sort of thing should not exist as it would fail to do its job properly - I bought an Audi S4 that was under 3 years old, and it looked like its previous owner had lovingly worked on the alloys with a toothbrush, never a good idea. Old toothbrushes are okay for scrubbing dirty unpainted car parts that normally don't get seen - as long as you don't leave them soaking in petrol!