I wonder how much of the thematic blindspot had to do with the technology of the day? The books were reportedly started on soon after The Hobbit (1939ish), before flight was the dominant theater for war.

Even though the books were not published until after WW2, when aerial warfare became the new front of war, it seems that Tolkien's main experience in war was the hideous ground fighting in WWI, and he himself rebuffed attempts to find parallels to WW2 in the LOTR series.

So while the concept of aerial war was not unknown, it may not have been the dominant thinking in Tolkien's time. And so, the idea that something pivotal could be conducted by just flying in by night was just easy to overlook -- not as in, he didn't think about it at all, just that it wasn't really worth a serious mention. Yes, obviously Tolkien lived through the part of human history in which the A-Bomb was dropped (though the books were on their way to being written at that point). But he's someone with a boots-on-the-ground worldview staging his book in the fantasy world in which flight is often not naturally experienced by the protagonists.