ARM managed to take the entire mobile market because those low power consumption ARM SoCs are everywhere, you can buy a tiny ARM board running Linux for $50 or less and there are thousands of sellers on ebay.com/taobao.com. However, it is a completely different situation when it comes to ARM for data centres - it is damn hard to find any 64bit ARM platform that allow you have say 32GBytes RAM and a few PCIE slots at reasonable price (e.g. comparable to Xeons). Not talking about cloud, I want 3 such ARM machines sitting in my home lab working 24/7 at 100% load for me and I need to play on the hardware side a little bit (e.g. try different SSDs).
As of writing, most of those devboards you can see on the market come with an application processor without modem. The ones that really see huge volumes are the ones with integrated modems, e.g. MTK processors with modem or the Snapdragon series, interestingly you are not going to get any of them for $50.
Do you know why no desktop software is optimized for Xeon Phi, SPARC, POWER9? Because almost nobody plays with them.