I think it's because it's the pure distillation of Trump. The man doesn't know McKinley from a hole in his ass. Here was his train of thought:
1. I got shot by my own supporter
2. I desperately need this to reflect well on me somehow
3. I've got it -- only "significant presidents get shot" (he said that -- not sure about what word he used for significant but that's the idea)
4. Trump to aide: give me a list of the presidents who were shot.
5. Who the fuck is James Garfield, or William McKinley?
6. McKinley had to have been a great president. He was the greatest. I'm saying that because I desperately need it to be true that only great presidents get shot.