Well you can buy 16GB server modules (ECC Ram), but I dunno if they make consumer ram in that size. In any case, it probably has something to do with the memory controller design. All of the 4 slot consumer motherboards I've ever seen only work with 32GB total (8 per slot). There's no real inherit architectural reason why this would be the case though. But, If the motherboard says it only supports 16, then it only supports 16. They wouldn't arbitrarily limit something like that. Memory controller configuration for DIMMs can be pretty nuanced depending on the design (this slot must be filled, this slot must have the smaller amount of memory, etc).
I wouldn't go and buy DDR3 2400. It's unlikely that you'd notice a memory bottleneck (or difference) in real world usage patterns.
EDIT: also are you actually using the 16GB anyway such that it would matter if you had 32. I don't find that I ever do even when running VMs and such.