Why does a motherboards maximum memory change from UDIMM to RDIMM

You've kind of answered this yourself, by the use of buffering - by using buffers memory operations don't have to be precisely synchronised allowing for more memory per channel. It's why registered memory is more expensive, it allows more memory per channel.

Chopper3 noted that registered memory allows one to install more DIMMs per memory channel. The other half of the equation is the amount of memory per DIMM. Supermicro (the mobo vendor) tested 16 GB registered DIMMs, and 2 GB unbuffered DIMMs. That isn't to say that larger unbuffered DIMMs won't work, only that supermicro didn't bother testing larger ones.

But, the final aspect is that one shouldn't discount documentation changes. Per the supermicro website (http://www.supermicro.com/products/motherboard/QPI/5500/X8DTH-6F.cfm), the UDIMM max is 48 GB (4GB DIMMs).