Skylake isn't revolutionary over Broadwell, offering only 5% gain in CPU performance at the same clock frequency. At the same time, the Broadwell offers GT3e integrated graphic card (e stands for the 128MB eDRAM cache), the highest performance of all intel GPUs. There is no Skylake SKU offers integrated GT3e at present. I think this is the main reason why Apple opt for Broadwell on 21-inch iMac.
Clearly, Apple doesn't want to use dGPU on the 21-inch model. So when Skylake might be available on 21-inch iMac depends on when intel wish to offer Skylake with a powerful integrated GPU.
Personally, I wouldn't care about the difference between Broadwell and Skylake. It is the lack of discrete GPU and the 5400rpm hard drive that make the 21-inch less favorable. If you absolutely need the 21-inch form factor, you should get it now. But be sure to upgrade to a SSD.