Goodness knows. I've said before that if it was actually possible to infect someone with prostate cancer, I'd suspect Kenny MacAskill (or his predecessor, since the diagnosis was actually made in 2008) of doing just that. It's the most convenient diagnosis since Socrates swallowed the hemlock.
Since it's not actually possible to infect anyone with prostate cancer, and since (despite Karol Sikora being pretty dodgy) I don't seriously doubt the diagnosis, I fall back on the procrastination theory.
Keep dragging your heels and putting off the moment when something has to happen, and hope that either something handy comes up, or you're out of office before the proverbial hits the fan.
In this case, something handy came up.