This is one of the FAQs in all my interactions with IT administrators. Here is the reason

As everyone knows 32 bit computers have a memory addressing limit of 4GB maximum. Since IT departments are looking at virtualization as a tool for consolidating multiple servers, this 4GB limit would not really help them. In order to achieve any meaningful consolidation of today’s and more importantly tomorrow’s workloads we need more than 4GB memory on Hyper-V enabled machines. Hence Microsoft decided to focus on x64 platform to develop and support Hyper-V.

Nevertheless there are scenarios where 32 bit hardware is available and need to be utilized. I suggest those customers to take advantage of Microsoft Virtual Server 2005 R2 SP1