What is the Architecture Limitation vSphere Replication have to Not to Support Physical Mode RDM?

When it comes to the way physical RDM (pRDM) works, these two bullet points from the KB article sum it up:

• Physical mode specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software.
• VMkernel passes all SCSI commands to the device, with one exception - The REPORT LUNs command is virtualized, so that the VMkernel can isolate the LUN to the owning virtual machine. Otherwise, all physical characteristics of the underlying hardware are exposed.

There is a vSphere Replication (VR) agent built into vSphere that performs a few functions. In particular, it tracks the writes to the virtual machine. When a replication cycle occurs, the changed data is replicated to the target location. The VR agent must be able to "see" these writes to track them. Since physical mode passes all SCSI commands directly to the device, the VR agent cannot track these changes for replication. Because of this Physical Mode RDM is not supported by vSphere Replication.

If you want to Know more about the Difference between Physical Mode RDM and Virtual Mode RDM check this KB