It's possibly a little more complicated than "just a spark plug" - Ferrari have boosted their engine power despite the new oil-burning regulations. This means that the cylinder heads are running a fraction hotter (the old-spec oil-burners ran cooler anyway) which may well have a knock-on to things like spark plugs. It may well only be a matter of a few degrees difference, but when you're already on the knife-edge of reliability, that might be the thing that tips you over.
Note that Hamilton also had a spark plug(s?) changed in final practice, and I recall him having them changed at the Austrian GP during practice. These are obviously far more critical than we realised and more prone to breakage - the way it was presented yesterday was that this is completely unheard of and a totally freak occurrence, which appears not to be the case on closer examination. Accordingly, the Ferrari team really should have been on top of the situation; this is yet another team failure.