It's debatable whether ARM can keep up with custom cores. Both the A15 and A57 designs have been underwhelming from a performance per watt point of view. The A57-based Snapdragon 810 has suffered from thermal issues: http://arstechnica.com/gadgets/2015/04/in-depth-with-the-sna....
Qualcomm is big, but Intel, Apple, and Samsung are all targeting custom cores in this space, and they may reasonably feel like doubling-down on their LTE technology and not trying to compete in that space.
Samsung has not yet used their own core, but their custom 64-bit ARM core is at an advanced stage of development: http://www.kitguru.net/components/cpu/anton-shilov/samsungs-...
Qualcomm is big, but Intel, Apple, and Samsung are all targeting custom cores in this space, and they may reasonably feel like doubling-down on their LTE technology and not trying to compete in that space.