Why we say it

I heard myself saying something (to myself) this morning, thinking “That’s the best thing since flush plumbing.”

I remember people using that expression when I was young, but it seems that most people around here don’t use that same one. The one more commonly used seems to be, “That’s the best thing since sliced bread.”

I googled that expression when I got to the office, and I read lots of variations of the same “idiom.” Some of them were not very nice, but others were more updated. Most of them compared the best thing to something more modern, like the internet or a smartphone.