For a very simple reason; if you are serious about Microsoft technologies you should get very familiar with the built-in virtualization in Server 2008.

Although the basic concepts are the same regardless of the fact of using Hyper-V, VMware, Virtual Box or any other virtualization environment, their practical usage varies from one platform to another.

So, IMHO, anyone getting a Microsoft certification should focus primarily on Hyper-V but obviously with an eye on the other options too.