Why has server processors often lower clockrate than consumer cpu?

Which server processors are you referring to? Server processors usually are available at the same speed as their desktop equivalent. As an example, there are no differences between the i7-980X and the X5680: http://ark.intel.com/Compare.aspx?ids=47932,47916,

server processors are usually similarly clocked to their desktop counterparts, but some server scenarios call for more cores, so you will find the more cores a part has the lower it is clocked to stay within a power cieling, since servers are susally not just built for brute force but also power efficiency

I was not really referring to any special CPU. From observations I was under some impression that server CPU's often was of a lower clockrate, and my first question was if that was in fact true.

You mean that it is common for such CPU's to have lower speed then?

That's true when referring to low voltage processors only, e.g. Xeon L5640 where an equivalent to the i7-980X doesn't exist. The X5680 runs just as hot as the i7-980X.

If looking at a very common enterprise server, the HP ProLiant DL380 G6, it could be equipped with some different processor types.

Some of them as low as Intel Xeon E5520 at 2.26 Ghz or even E5504 at 2.0 Ghz.

Link: http://h10010.www1.hp.com/wwpc/us/en/sm/WF25a/15351-153...

Other much more expensive variant has X5570 at 2.93 Ghz.