I believe that the key shift occurred during the Revivial of the 1960s and early 70s, and the reason for the change was that lots of people wanted to play scots/irish fiddle tunes, for which D is a more common key. You'd have to go back to early editions of DPN to find any possible hard information. Or you could question some of the folks who were there -- like Howard Rugg, Robert Force, Bonnie Carol, Lois Hornbostle, etc.
IMHO most of today's D diehards are that way simply because that's the way they were taught and they were (unfortunately) taught that DAd is the only 'right' way to do things . They have little or no desire to dig into the background of the instrument, and no desire to learn to re-tune to become more involved with the instrument, because it takes them out of their 'comfort zone'.
Most of the books today are in D for those same reasons. For those of us who use different keys and understand that the key makes no difference, it's all good. Personally I've been playing in B a lot lately as it's easier on my voice than D or falsettoG. For instrumentals in a high noise environment I personally love Gdd or Ggg as that note seems to cut through a lot of background buzz.
DAA is the proper notation, also DAC (you tune down to the C not up to c from A), and Ddd or ddd, but not DDD which would be all bass strings.
Because the key note doesn't particularly matter, many people today simply refer to 1-5-5 or 1-5-7 or 1-5-8 or 1-8-8 tunings to indicate they they are note-independent.