|Not odd at all if you're familiar with Rolling Stone and the prevailing American music culture of the late 60's and 70's.|
Simply put, prog/symphonic/art rock, or whatever you want to call it, just didn't have as high a profile in American music culture as more blues based and mainstream pop/rock music. Prog was a more European thing, and most prog bands didn't achieve much more than cult status over here. Plus, there was a certain distain for anything prog in the American music media. Arguably the two biggest and most influential magazines of the time, Rolling Stone and Creem, rarely featured prog related bands or music, although Circus magazine did have the occasional article whenever certain bands released new albums or toured.
There was a story circulating a few years ago regarding the selection process for the RRHOF (not sure if it is actually true). The Moody Blues were apparently being considered for nomination, but Jann Wenner, RS co-founder & publisher, laughed off the suggestion. Seems he personally had no interest in the band and didn't view them as worthy of recognition.
The strange thing is, The Moody Blues did have enough of a following in North America to allow them to do reasonably well from a commercial standpoint, having a couple of No. 1 albums plus the fairly popular Days Of Future Past. Despite that, their representation in the media at the time was relatively low.
I do agree that they deserve the recognition, along with bands like King Crimson, Yes, and ELP, but it might take a little while for the powers that be to come around to that realization.