Why is it called Windows Server 2008 R2? Why not simply call it Windows Server 2010?

One guess: To remove the psychological barrier to upgrade. If you call it R2 then it looks like a Service Pack (which it is to some extent), whereas if you call it 2010 then it looks like a huge new version with massive incompatibilities, which will make people very reluctant to update.

R2 is a licensing thing. It has nothing to do with kernel changes or service packs or other such things.

Remember 2003? And then 2003 R2? 2003 R2 did NOT have any kernel changes. 2003 R2 was basically a bunch of add-ons, SOME of which were freely available from Microsoft, some were not. It did introduce some new technologies/advanced some existing ones and bump the schema up a version, but the KERNEL was the same.

Server 2008 R2 is radically different from Server 2008. There are MANY kernel changes, especially in the area of power management (core parking for one) and the ability to handle (with the appropriate version) 256 Processor Cores.

R2 provides a way of saying "Your CALs are still good". With R2, you buy the SAME CALs (the CALs don't need to be upgraded) that you did with the original version.

(FYI, I saw a presentation on 2003 Server by a Microsoft Rep who challenged the thought that Windows 7 was Vista R2 and upon hitting him with the logic based on kernel similarities, he explained why R2 exists).

Because it is not a NEW version, it is really a tidying up of the old version. Basically think of it as too many underlying changes for a service pack but too few obvious features to be a whole new product.

They've done this before with things like Win 98SE. Compare the reception that got to Win ME.

You have to wonder if Windows XP SP2 would have been called "Windows XP R2" as it was a pretty big re-write of the Windows XP code.

On the server side, whatever Microsoft do someone will complain. I suppose it does make it clearer to people that their CALs are still good, but hopefully people making major decisions on network operating systems are going a bit further than the name of new products when making their decisions. At the end of the day a rose by any other name...

I think it's a commercial move. Windows Server 2008R2 looks old if Microsoft releases a new version in the future. Let's say they'll release a Windows 2014, then Windows 2008 R2 looks like 6 years old (which isn't).