Just my opinion, but i think i can pretty well say why we have #7 shot. Back 100 plus years or so ago the list of available sizes were virtually unlimited, including many half sizes, & all those big ones which have been re-introduced in steel, but they were of course lead back then. Around the time of WWI the factories began "Streamlining" & reducing the offerings. The "Big" sellrs stayed, the slower sellers went, simple as that. Thus #7 stayed because a lot of folks were using it.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra