I can't tell you for sure what's the reason for this, but I guess it's related to having more than a single partition VMFS on the installation disk. Resizing the VMFS partition requires rewriting/reconfiguring of the partition table, which is even more complex with GPT due to the protective MBR at the end of the disk. I assume the reason why it is only supported for LUNs with a single VMFS partition is related to this complexity and also to the business/use case, where it's way more likely that a dedicated storage LUN is being resized than the local installation disk.
Also keep in mind that some RAID controllers (the vendor's drivers) currently do not support sizes >2TB (see e.g. http://kb.vmware.com/kb/2006942). What I usually do in such a case, is to add the disks to the RAID set and create an additional logical volume on the new disk space.
André
