There's some more explanation of why this is at http://bits.blogs.nytimes.com/2012/09/13/iphone-5-calls-data/ (thanks to [MacRumors](http://www.macrumors.com/2012/09/13/verizon-iphone-5-will-not-offer-simultaneous-voice-and-data/)).  This was a hardware decision.  Adding support would require adding a third antenna.  The relevant quote

> So why does Verizon’s Samsung Galaxy S III, a 4G LTE phone, juggle calls and data? Samsung added an extra antenna so that it pulls data from the 4G LTE network at the same time that it’s using another antenna to do voice, said Anand Shimpi, editor in chief of AnandTech.

> Then why didn’t Apple add another antenna? It actually already has two antennas in an effort to improve reception, and it would have had to add a third antenna just for Verizon and Sprint phones to give them simultaneous data and calls, Mr. Shimpi explained. Leaving that third antenna out allows Apple to simplify its manufacturing process of the iPhone for multiple carriers. Plus, in the next two years the 4G LTE network is supposed to evolve to support voice calls, which would render another antenna unnecessary later.