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It's quite possible that apple don't want to encourage people to buy it knowing they'll put broadwell in there in a few months.
 
I'd assume it's because they didn't get any of the other updates that the 13" got. Updated graphics, processor, SSD speed, battery, etc. If you're not updating anything else why bother disrupting your supply and manufacturing process just to update the trackpad?
 
Probably have a "bigger update" in store in the near future (which could mean up to 6 months).
 
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