Qualcomm differentiate themselves through custom CPU cores. They aren't giving up one of their competitive advantages.
QC competitive advantage is not the custom CPU, it's the ability to create the best possible SoC, the failure to understand that is exactly why people keep thinking that Intel will succeed because "it has better CPUs".
When ARM somewhat failed to deliver with the Cortex-A15, QC had Krait, which made them very competitive.
It seems the A57 is more competitive this time, so QC does not need its own CPU.
I don't believe one second that they could not design an ARMv8 Krait by the same timeframe they are getting the A57 + A53 integrated.
What they could not do is do something that would give them this competitive advantage, so they have chosen to focus on the overall SoC, because this is what they are selling.
And now AMD, NVIDIA, Broadcom, Apple, probably QC as well are all doing their own implementation, but they will keep comparing them with vanilla ARM cores (which they can access well before the availability in products as would happen with a competitor core), this is a very good way for them to make sure they are properly differentiating and not spending a fortune reinventing the wheel.
I'd go by first release in a product as being the most indicative, and by that it's been a pretty solid two year cadence between the flagship CPUs.
October 2008: Cortex-A8 (OMAP3530 in Archos 5 and other PMPs)
August 2010: Cortex-A9 (Tegra 2 in Toshiba AC100)
October 2012: Cortex-A15 (Exynos 5250 in Samsung Chromebook)
October 2014: Cortex-A57 (Exynos 5433 in Galaxy Note 4)
I expect we'll hear about the next ARM CPU soon.
Indeed Wikipedia gives the date of the announcement, but the delay between announcement and shipping has somewhat been reduced over the years.