It's less about word size and more about the underlying CPU architecture, i.e. ARMv8. See

http://en.wikipedia.org/wiki/ARMv8#ARMv8_and_64-bit

It's much easier to market 64-bit than ARMv8. But what really matters are a new instruction set, more registers and better SIMD.

You have to give credit to Apple as they are indeed the first one to come out with a commercial SOC based on this and the first to get the OS support added.