Those decisions are pretty arbitrary. Way back on iOS 5, the iPod Touch 4 got the update and the original iPad missed it, despite using the exact same processor and RAM.
Not back then. This was before "Retina" iPads, but the iPod Touch was Retina. You had a 960x640 screen vs a 1024x768 one. More pixels, but not so many more that it should block an OS update.