I think what you are seeing is the result of way Americans incorporate things into our consciousness - TV/Movies. I think that even if people still read as much "as they used to" (not a very quantifiable comment but you get the idea) seeing someone in some sort of motion picture really brings it home in people's minds. I will add at this point that since people are learning less and less about WWII, when they see some sliver of it on TV or in the movies that becomes their image of what WWII was. So now there are a lot of people to whom Saving Private Ryan, Band of Brothers, and The Pacific IS WWII. So now when one of those folks die (or even those just associated with the unit) it is like losing a movie star. If there were a movie about the 540th, which fought for 912 days I might add, perhaps there would be more of an outpouring from posters. As it is, those vets who die who were members of units that weren't the Rangers, Easy Co, and "the Marines" (i.e. infantry Marines who were on those Island campaigns) are not going to get the credit that the others get.
And as you state, this is not to take away from those gentlemen!! It is only to lament that there are others who are deserving of the same recognition.
Is it perhaps that Americans only have a certain amount of attention span and there are too many vets dying for people to be upset? Perhaps. That's just a feeling I have. We will leave the figuring out if that is right or not to the pollsters.