It might be just a strike of luck that the cat haven't been sick in so long. This spesific cat and sim didn't live in BB very long, at most 3 sim weeks. It seemed the cat had to be taken to the vet at least twice a week in that time. I do rotational play also, but tend to stay with a household for quite a long time, only switching when I get "tired" of them and need change. I don't play the households in any spesific order. Also, I only have 1 house with a pet (they are also not MY focus when playing), not counting any of the premades or randomly generated sims with pets (which I normally don't play anyway).
There might be some hidden triggers that make some pets more sick than others, or areas in the world that triggers sickness more often. Now with Seasons, climate could also play a role.