What are the reasons for these two very different post mount adapter designs?
While I can't find anything from Shimano themselves, this description is used in regards to the second one on a number of websites selling them (so perhaps it is in Shimano's sales literature?):


  2nd version of adaptor, lighter and better hose routing


Since this is the best I can find, I'm posting it here as an answer, but by all means if someone else finds more details post that up.