The biggest reason for that is that Apple, Qualcomm, and Samsung have designed their own custom cores, rather than using ARM's license-able ones. So most of the phones you see, instead of using Cortex A72, use Snapdragon 820 or Exynos M1. (Huawei's Kirin 950 uses A72 cores, though.)
One of the more interesting bits about the A73 announcement is that they're offering a new licensing level for SoC makers to customize their cores starting from an A73 base, and that Qualcomm has taken that license; so the next-gen Qualcomm SoC won't have the A73 name, but may have a lot of its tech.
Unlike, say, AMD, Intel or Apple, ARM does not make or market CPUs to consumers. They design and market CPU IP to SOC makers. The road from inception to actual product for a typical SOC is longer than a year, and from that it takes time for device manufacturers to actually put the SOCs into devices and sell them to you.
This naturally means that it takes quite a lot of time for ARM cores to go from the time you first hear about them (when they are being marketed to SOC makers) to being as close to release as the point when you typically hear about other CPU cores (when they are being marketed to you).
Especially with ARM being an IP vendor. There's usually a 2 year period between new core announcement and shipping, although this time it may be a little shorter.
