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Apple’s iPhone 5 got bigger than its predecessor, and some wonder why the company didn’t make room inside for a technology called near-field communications (N.F.C.), which enables devices to exchange information wirelessly over very short distances. This feature could make paying for things with a phone less of a hassle. (You’d swipe the phone over a device at the checkout counter.) It turns out that getting N.F.C. to work right with the new design of the iPhone wasn’t even possible.

Why not? Nokia’s new Lumia 920 smartphone supports N.F.C., and some Android smartphones do as well. The explanation is actually quite simple. The iPhone 5’s all-aluminum-and-glass body would block information from being transmitted to a terminal, according to Will Strauss, an analyst of Forward Concepts, a research firm that follows digital signal processing and chips.

“N.F.C. employs lower-frequency operation than cellular, requiring a longer antenna,” Mr. Strauss said. “That antenna is often wrapped around the battery in some cellphones, but a metal back shields any radio waves from reaching a nearby data terminal. Only plastic, Kevlar or similar backings will allow the radio connection for mobile payments. Clearly, Apple chose beauty over functionality with its aluminum back.”

Another explanation is that Apple simply didn’t want to add N.F.C. yet, because the company doesn’t do something unless it feels the market is there. For instance, it never released a cheap Apple netbook because it didn’t have faith in that market. Other times it is way ahead of the marketplace — witness the iPhone itself, which radically changed the cellphone market. Though it’s been gaining momentum, mobile payments technology is still immature, with plenty of hurdles to clear before it becomes more widely adopted. There are numerous competing technologies and it isn’t clear which one or ones — there can’t be many — will become dominant.