My take on it is this – the Shoah is in a category of tragedy all by itself.
For a different take on this, take a look at the Artscroll Kinnos on #25 (the one I mentioned earlier) where the author explains, in better words than I ever could, how all tragedies which r”l befall Klal Yisroel are actually an outgrowth of the churban habayis.
The very broad nature of the tragedy, R’ Moshe writes in the teshuva which I mentioned, is an indication that it is a product of our being in golus.
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The story is told about the Bobover Rebbe Zt”l that he felt that anyone in our dor is unworthy of composing a kinnah, until he saw in a sefer that anyone can. Possibly, some gedolim agree with his initial thought.
In Halichos Shlomo (in the section on Tisha B’av, I don’t have it in front of me for a precise citation), it is described how and why one year R’ Shlomo Zalman said R’ Weissmandl’s kinnah, but that the practice was discontinued.