Unified Parallel C is definitely oriented towards supercomputer markets, where ARM is not a major player and never has been. Supercomputers used to be heavily based on POWER, MIPS, and Sparc, but around 2003 or so, x86 dominated the market. According to TOP 500, something like 80+% of supercomputers are running Intel processors (not even AMD is really competitive anymore), with POWER making up most of the non-x86 crowd (there's a handful of Sparcs running around, plus some Chinese home-grown architectures).

Something else that's happened in the past 6 years or so has been the rise of NVidia boards to get high TFLOPS counts. If I look at the mailing lists, it seems interest in UPC drops off somewhere around 2011-2012, and I don't think it's a coincidence that the rise of GPGPU in HPC applications has something to do with it.