Apple obviously doesn’t use Qualcomm Snapdragon SoCs so enabling LTE on the iPhone isn’t possible using Qualcomm baseband unless you make the phone’s PCB larger (which Apple obviously wasn’t going to do). Note that no one else seems to deliver a single chip LTE + 1x/WCDMA voice solution either, so this isn’t just a Qualcomm limitation.
Basically because as magical as Apple makes engineering seem, it just wasn’t physically possible and won’t be until at least 2012 sometime. And even then, will LTE even be rolled out in enough markets?
via AnandTech – Why No LTE iPhone 5 in 2011? Blame 28nm Maturity, Check Back In Q2/Q3 2012.