Well, seems pretty obvious to me:
Smoking creates lots of particulates in the air which settle on all things and coat them with a sticky sludge. The Wii (like other optical media readers) uses a ridiculously-sensitive laser and laser-pickup to read the disc. When this gets dirty, you get problems.
Cigarette smoke is harder to clean off than other forms of dust. A little brushing often isn't enough, and you often have to use a light solvent like alcohol. I speak from 15+ years in the IT industry, much of that fixing computers.
Just another way smoking is bad (aside from the obvious health issues).