To prevent third parties from bothering to make "17"-only" ExpressCard devices and leaving 15" MBP users out cold?
There aren't very many Mac-compatible peripherals anyway. Even though the the 54mm version is backwards-compatible with the 34mm version, I doubt many companies would bother making 54mm ExpressCards if their entire potential customer base was 17" MBP owners. It simply wouldn't make sense to include a larger slot because it would never be used.
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