I think the real reason has to do with 1) there was an existing, organized pro-Palestinian movement that had experience protesting; 2) many organizations on the left saw the Israel-Gaza conflict as fitting very nicely into their larger anti-imperialist ideology in a way that other conflicts don't; 3) everyone more-or-less knows where Israel is on the map and has some familiarity with it; 4) there were a lot of really shocking images and video from Gaza
https://www.youtube.com/watch?v=DpiW-r-zfW8
I've seen a bunch of protesters about the war in Yemen outside the bomb factories around me.