AnandTech's Brian Klug has a detailed post about Apple's decision, drawing on iPhone 5 test results posted by the FCC.
"[A]t a high level this is a design decision which makes the phone as small and light as it is (it really is light, almost alarmingly so) and enables it to support a wide number of LTE bands, rather than some major oversight like I've seen it portrayed," Klug writes.
Here's the background: Apple has announced two iPhone 5 hardware models, A1428 and A1429, with three different provisioning configurations. The hardware differences "accommodate a number of different LTE bands between the two." The software differences in how the two models are configured have to do with the initial provisioning of the phones and probably the version of Qualcomm's Advanced Mobile Subscriber Software (running on the baseband chip) that gets loaded at boot time, according to Klug.
With this approach, "Apple is able to support no fewer than 8 LTE bands with largely the same hardware -- the same display, chassis, battery, form factor, and PCB outline (different power amplifiers and filters are required), and roughly the same exterior antennas (gain is different on the primary bottom antenna between the two models, no doubt they're tuned differently)," Klug says. "Previously most handsets I've seen have been destined to work only on a single carrier, and thus implement at most one or two LTE bands." (You can check Apple's official list of supported LTE bands.)
According to Klug, today's simultaneous voice-data implementations -- SVLTE (simultaneous voice and LTE) and SVDO (simultaneous voice and EVDO) -- require a three-antenna solution and two transmit radio frequency chains for CDMA phones. Apple's approach avoids all that extra space-demanding hardware, because it sticks with widely used circuit-switched fallback (CS-FB) -- "This quite literally means you drop from 4G LTE to 3G WCDMA (where voice and data are already multiplexed) for the call, then hand back up to LTE when you're finished," says Klug.
"What it really boils down to is that by using this single Tx [transmit] chain, Apple is able to support a ton of LTE bands ... and also do it without making the iPhone very large," Klug says. "Moving to an architecture that works with SVDO and SVLTE would require an additional transmit path and antenna, and incur a size and weight penalty."
No LTE roaming either