Why ARM CPUs are not used in servers and desktops?
From what I see:
Software compatibility (for most people, a desktop PC that can't run x86 software is a definitive no-no).
Even the fastest ARM CPUs are often slower than x86 CPUs. From here (which links to this Phoronix article):
In a nutshell, the system with a 1 GHz TI OMAP 4460 dual core processor came out ahead of the netbook with a 1.6 GHz Intel Atom N270 chip in some of the benchmarks, and came out behind in others.
Power consumption is not really the biggest concern on a desktop computer.
Though ARM is starting to be used on servers, e.g. Calxeda:
Founded in 2008 and funded with a $48M investment from ARM and seasoned venture capital investors in 2010, Calxeda provides revolutionary efficiency to the data center. Leveraging ultra-low power technology from ARM, servers built on Calxeda’s silicon and software platform consume a fraction of the power and space of today’s best-in-class servers, enabling data centers to realize significant reduction in capital and operating expenses.
And recently AMD announced plans to make ARM-based Opterons
CHIP DESIGNER AMD has announced that it will release ARM based server chips under its Opteron brand in 2014.
In addition to what Renan said, there weren't 64-bit ARM parts in 2012. Since servers have needed more than 4 GB for some time, this is a show stopper. However as pointed out by Igor Skochinsky below, it isn't quite that simple
Though ARM is catching up the pace quickly, IMO:
Intel still manages to do better on single core performance, better memory bandwidth, larger cache design, and reasonable power consumption these days. One of the current i7 core can outperform best single ARM core by 3~10x(depending on application).
And i7/xeon is already 4/6 core per chip + hyper threading. you will need 64/128/256 ARM core to match up.