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.